Add support for gitattributes

Runs git check-attr to get the files git attributes, and treats the file
as binary if the attribute value is "unset", matching the built-in git diff.

Fixes #466

Helped-by: @anuramat (#466 (comment))

Author: WGH-

src/git.rs

@@ -0,0 +1,63 @@
+use std::path::Path;
+use std::process::Command;
+
+/// Corresponds to the diff attribute. See man gitattribute.
+pub(crate) enum DiffAttribute {
+    Set,
+    Unset,
+    Unspecified,
+    Other,
+}
+
+impl From<&str> for DiffAttribute {
+    fn from(s: &str) -> Self {
+        match s {
+            "set" => Self::Set,
+            "unset" => Self::Unset,
+            "unspecified" => Self::Unspecified,
+            _ => Self::Other,
+        }
+    }
+}
+
+/// Runs `git check-attr diff` to get the diff attribute of the path. Returns
+/// [`Option::None`] when either `git` is not available, file is not inside git
+/// directory, or something else went wrong.
+pub(crate) fn check_attr(path: &Path) -> Option<DiffAttribute> {
+    let res = Command::new("git")
+        .args(["check-attr", "diff", "-z", "--"])
+        .arg(path)
+        .output();
+
+    match res {
+        Ok(output) => {
+            // Either git is not available, or file is outside git directory.
+            if !output.status.success() {
+                debug!("git check-attr exited with status {}", output.status);
+                return None;
+            }
+
+            // The output format is "path" "attribute name" "value". We
+            // specified both path and attribute name explicitly,
+            // so we only need value here.
+            let stdout = &output.stdout;
+            let value = stdout.split(|&b| b == b'\0').nth(2);
+            match value {
+                None => {
+                    warn!("malformed git check-attr output {stdout:#?}");
+                }
+                Some(value) => match std::str::from_utf8(value) {
+                    Ok(s) => return Some(s.into()),
+                    Err(err) => {
+                        warn!("invalid diff attribute value: {err}");
+                    }
+                },
+            }
+        }
+        Err(err) => {
+            warn!("failed to execute git: {err}");
+        }
+    }
+
+    None
+}

src/main.rs

@@ -38,6 +38,7 @@ mod diff;
 mod display;
 mod exit_codes;
 mod files;
+mod git;
 mod hash;
 mod line_parser;
 mod lines;
@@ -68,6 +69,7 @@ use crate::files::{
     guess_content, read_file_or_die, read_files_or_die, read_or_die, relative_paths_in_either,
     ProbableFileKind,
 };
+use crate::git::{check_attr, DiffAttribute};
 use crate::parse::guess_language::language_globs;
 use crate::parse::guess_language::{guess, language_name, Language, LanguageOverride};
 use crate::parse::syntax;
@@ -405,8 +407,11 @@ fn diff_file(
     let (mut lhs_src, mut rhs_src) = match (
         guess_content(&lhs_bytes, lhs_path, binary_overrides),
         guess_content(&rhs_bytes, rhs_path, binary_overrides),
+        check_attr(Path::new(display_path)),
     ) {
-        (ProbableFileKind::Binary, _) | (_, ProbableFileKind::Binary) => {
+        (ProbableFileKind::Binary, _, _)
+        | (_, ProbableFileKind::Binary, _)
+        | (_, _, Some(DiffAttribute::Unset)) => {
             let has_byte_changes = if lhs_bytes == rhs_bytes {
                 None
             } else {
@@ -425,7 +430,7 @@ fn diff_file(
                 has_syntactic_changes: false,
             };
         }
-        (ProbableFileKind::Text(lhs_src), ProbableFileKind::Text(rhs_src)) => (lhs_src, rhs_src),
+        (ProbableFileKind::Text(lhs_src), ProbableFileKind::Text(rhs_src), _) => (lhs_src, rhs_src),
     };
 
     if diff_options.strip_cr {