Add runCommand function to Go template syntax

This makes it possible to use date and time in initial values like this:

```yaml
initialValue: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

I want to use this to configure my BranchPrefix like this:

```yaml
git:
  branchPrefix: 'ruudk/{{ runCommand "date +\"%Y/%-m\"" }}/'
```

Author: ruudk

Diff for: docs/Config.md

@@ -1008,6 +1008,15 @@ git:
   branchPrefix: "firstlast/"
 ```
 
+It's possible to use a dynamic prefix by using the `runCommand` function:
+
+```yaml
+git:
+  branchPrefix: "firstlast/{{ runCommand "date +\"%Y/%-m\"" }}/"
+```
+
+This would produce something like: `firstlast/2025/4/`
+
 ## Custom git log command
 
 You can override the `git log` command that's used to render the log of the selected branch like so:

Diff for: docs/Custom_Command_Keybindings.md

@@ -320,6 +320,24 @@ We don't support accessing all elements of a range selection yet. We might add t
   command: "git format-patch {{.SelectedCommitRange.From}}^..{{.SelectedCommitRange.To}}"
 ```
 
+We support the following functions:
+
+### Quoting
+
+Quote wraps a string in quotes with necessary escaping for the current platform.
+
+```
+git {{.SelectedFile.Name | quote}}
+```
+
+### Running a command
+
+Runs a command and returns the output. If the command outputs more than a single line, it will produce an error.
+
+```
+initialValue: "username/{{ runCommand "date +\"%Y/%-m\"" }}/"
+```
+
 ## Keybinding collisions
 
 If your custom keybinding collides with an inbuilt keybinding that is defined for the same context, only the custom keybinding will be executed. This also applies to the global context. However, one caveat is that if you have a custom keybinding defined on the global context for some key, and there is an in-built keybinding defined for the same key and for a specific context (say the 'files' context), then the in-built keybinding will take precedence. See how to change in-built keybindings [here](https://github.com/jesseduffield/lazygit/blob/master/docs/Config.md#keybindings)

Diff for: pkg/gui/controllers/helpers/refs_helper.go

@@ -2,8 +2,6 @@ package helpers
 
 import (
 	"fmt"
-	"strings"
-
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/commands/git_commands"
 	"github.com/jesseduffield/lazygit/pkg/commands/models"
@@ -12,6 +10,8 @@ import (
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 	"github.com/jesseduffield/lazygit/pkg/utils"
 	"github.com/samber/lo"
+	"strings"
+	"text/template"
 )
 
 type IRefsHelper interface {
@@ -329,7 +329,27 @@ func (self *RefsHelper) NewBranch(from string, fromFormattedName string, suggest
 	)
 
 	if suggestedBranchName == "" {
-		suggestedBranchName = self.c.UserConfig().Git.BranchPrefix
+		var err error
+
+		suggestedBranchName, err = utils.ResolveTemplate(self.c.UserConfig().Git.BranchPrefix, nil, template.FuncMap{
+			"runCommand": func(command string) (string, error) {
+				output, err := self.c.Git().Custom.RunWithOutput(command)
+				if err != nil {
+					return "", err
+				}
+
+				output = strings.TrimRight(output, "\r\n")
+
+				if strings.ContainsAny(output, "\r\n\t ") {
+					return "", fmt.Errorf("command output contains whitespace characters: %s", output)
+				}
+
+				return output, nil
+			},
+		})
+		if err != nil {
+			return err
+		}
 	}
 
 	refresh := func() error {

Diff for: pkg/gui/services/custom_commands/handler_creator.go

@@ -3,16 +3,15 @@ package custom_commands
 import (
 	"errors"
 	"fmt"
-	"strings"
-	"text/template"
-
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/config"
 	"github.com/jesseduffield/lazygit/pkg/gui/controllers/helpers"
 	"github.com/jesseduffield/lazygit/pkg/gui/style"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 	"github.com/jesseduffield/lazygit/pkg/utils"
 	"github.com/samber/lo"
+	"strings"
+	"text/template"
 )
 
 // takes a custom command and returns a function that will be called when the corresponding user-defined keybinding is pressed
@@ -247,6 +246,19 @@ func (self *HandlerCreator) getResolveTemplateFn(form map[string]string, promptR
 
 	funcs := template.FuncMap{
 		"quote": self.c.OS().Quote,
+		"runCommand": func(command string) (string, error) {
+			output, err := self.c.Git().Custom.RunWithOutput(command)
+			if err != nil {
+				return "", err
+			}
+			output = strings.TrimRight(output, "\r\n")
+
+			if strings.Contains(output, "\r\n") {
+				return "", fmt.Errorf("command output contains newlines: %s", output)
+			}
+
+			return output, nil
+		},
 	}
 
 	return func(templateStr string) (string, error) { return utils.ResolveTemplate(templateStr, objects, funcs) }

Diff for: pkg/integration/tests/branch/new_branch_with_prefix_using_run_command.go

@@ -0,0 +1,33 @@
+package branch
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var NewBranchWithPrefixUsingRunCommand = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Creating a new branch from a commit with a default name",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Git.BranchPrefix = "myprefix/{{ runCommand \"echo dynamic\" }}/"
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			EmptyCommit("commit 1")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit 1").IsSelected(),
+			).
+			SelectNextItem().
+			Press(keys.Universal.New).
+			Tap(func() {
+				branchName := "my-branch-name"
+				t.ExpectPopup().Prompt().Title(Contains("New branch name")).Type(branchName).Confirm()
+				t.Git().CurrentBranchName("myprefix/dynamic/" + branchName)
+			})
+	},
+})

Diff for: pkg/integration/tests/custom_commands/run_command.go

@@ -0,0 +1,40 @@
+package custom_commands
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var RunCommand = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Using a custom command that uses runCommand template function in a prompt step",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().CustomCommands = []config.CustomCommand{
+			{
+				Key:     "a",
+				Context: "localBranches",
+				Command: `git checkout {{.Form.Branch}}`,
+				Prompts: []config.CustomCommandPrompt{
+					{
+						Key:          "Branch",
+						Type:         "input",
+						Title:        "Enter a branch name",
+						InitialValue: "myprefix/{{ runCommand \"echo dynamic\" }}/",
+					},
+				},
+			},
+		}
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Focus().
+			Press("a")
+
+		t.ExpectPopup().Prompt().
+			Title(Equals("Enter a branch name")).
+			Type("three").
+			InitialText(Contains("myprefix/dynamic/")).
+			Confirm()
+	},
+})