Add configuration support for backend selection

Add git_backend field to ProtofetchConfig and support loading from
config file or PROTOFETCH_GIT_BACKEND environment variable.

Author: rtimush

src/config.rs

@@ -5,14 +5,16 @@ use config::{Config, ConfigError, Environment, File, FileFormat};
 use log::{debug, trace};
 use serde::Deserialize;
 
-use crate::model::protofetch::Protocol;
+use crate::{git::backend::GitBackendType, model::protofetch::Protocol};
 
 #[derive(Debug)]
 pub struct ProtofetchConfig {
     pub cache_dir: PathBuf,
     pub default_protocol: Protocol,
     pub jobs: Option<usize>,
     pub copy_jobs: Option<usize>,
+    pub git_backend: GitBackendType,
+    pub git_executable: Option<String>,
 }
 
 impl ProtofetchConfig {
@@ -28,6 +30,8 @@ impl ProtofetchConfig {
             default_protocol: raw_config.git.protocol.unwrap_or(Protocol::Ssh),
             jobs: raw_config.jobs,
             copy_jobs: raw_config.copy_jobs,
+            git_backend: raw_config.git.backend.unwrap_or_default(),
+            git_executable: raw_config.git.executable_path,
         };
         trace!("Loaded configuration: {:?}", config);
 
@@ -55,6 +59,8 @@ struct CacheConfig {
 #[derive(Default, Debug, Deserialize, PartialEq, Eq)]
 struct GitConfig {
     protocol: Option<Protocol>,
+    backend: Option<GitBackendType>,
+    executable_path: Option<String>,
 }
 
 impl RawConfig {
@@ -146,7 +152,11 @@ mod tests {
             config,
             RawConfig {
                 cache: CacheConfig { dir: None },
-                git: GitConfig { protocol: None },
+                git: GitConfig {
+                    protocol: None,
+                    backend: None,
+                    executable_path: None,
+                },
                 jobs: None,
                 copy_jobs: None,
             }
@@ -158,7 +168,10 @@ mod tests {
         let env = HashMap::from([
             ("PROTOFETCH_CACHE_DIR".to_owned(), "/cache".to_owned()),
             ("PROTOFETCH_GIT_PROTOCOL".to_owned(), "ssh".to_owned()),
+            ("PROTOFETCH_GIT_BACKEND".to_owned(), "cli".to_owned()),
         ]);
+        // Note: `git.executable_path` is a config-file-only setting; the
+        // `Environment` source's `_` separator would split a two-word key.
         let config = RawConfig::load(None, Some(Default::default()), Some(env)).unwrap();
         assert_eq!(
             config,
@@ -167,7 +180,9 @@ mod tests {
                     dir: Some("/cache".into())
                 },
                 git: GitConfig {
-                    protocol: Some(Protocol::Ssh)
+                    protocol: Some(Protocol::Ssh),
+                    backend: Some(GitBackendType::Cli),
+                    executable_path: None,
                 },
                 jobs: None,
                 copy_jobs: None,
@@ -197,6 +212,8 @@ mod tests {
 
                 [git]
                 protocol = "ssh"
+                backend = "cli"
+                executable_path = "/usr/bin/git"
             }),
             Some(env),
         )
@@ -208,7 +225,9 @@ mod tests {
                     dir: Some("/cache".into())
                 },
                 git: GitConfig {
-                    protocol: Some(Protocol::Ssh)
+                    protocol: Some(Protocol::Ssh),
+                    backend: Some(GitBackendType::Cli),
+                    executable_path: Some("/usr/bin/git".to_owned()),
                 },
                 jobs: None,
                 copy_jobs: None,