Fix: Always Use Default Diff

This fix resolves issues that arise when the user has configured an
alternative diff provider through diff.external. It also ensures that
the default prefix setting is consistently used, allowing `revgrep` to
operate correctly regardless of user settings.

revgrep.go

@@ -366,7 +366,7 @@ func GitPatch(revisionFrom, revisionTo string) (io.Reader, []string, error) {
 	}
 
 	if revisionFrom != "" {
-		cmd := exec.Command("git", "diff", "--color=never", "--relative", revisionFrom)
+		cmd := exec.Command("git", "diff", "--color=never", "--no-ext-diff", "--default-prefix", "--relative", revisionFrom)
 		if revisionTo != "" {
 			cmd.Args = append(cmd.Args, revisionTo)
 		}
@@ -384,8 +384,7 @@ func GitPatch(revisionFrom, revisionTo string) (io.Reader, []string, error) {
 	}
 
 	// make a patch for unstaged changes
-	// use --no-prefix to remove b/ given: +++ b/main.go
-	cmd := exec.Command("git", "diff", "--color=never", "--relative", "--")
+	cmd := exec.Command("git", "diff", "--color=never", "--no-ext-diff", "--default-prefix", "--relative", "--")
 	cmd.Stdout = &patch
 	if err := cmd.Run(); err != nil {
 		return nil, nil, fmt.Errorf("error executing git diff: %w", err)
@@ -400,7 +399,7 @@ func GitPatch(revisionFrom, revisionTo string) (io.Reader, []string, error) {
 
 	// check for changes in recent commit
 
-	cmd = exec.Command("git", "diff", "--color=never", "--relative", "HEAD~", "--")
+	cmd = exec.Command("git", "diff", "--color=never", "--no-ext-diff", "--default-prefix", "--relative", "HEAD~", "--")
 	cmd.Stdout = &patch
 	if err := cmd.Run(); err != nil {
 		return nil, nil, fmt.Errorf("error executing git diff HEAD~: %w", err)