Name a diff line's file in the repo's terms in one place

A diff line carries the absolute path of its file, and both panels
acting on such a line turn it into the repo-relative one git speaks
themselves. Opening a line in a pull request needs that path too, to
name the file to GitHub by it.

Extract repoRelativePath, and have both panels use it. The files panel
gains the check for a path outside the repo that the other one had; a
path it used to pass on matches no file of the working tree either.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/gui/controllers/commit_diff_actions.go

@@ -2,8 +2,6 @@ package controllers
 
 import (
 	"fmt"
-	"path/filepath"
-	"strings"
 
 	"github.com/jesseduffield/generics/set"
 	"github.com/jesseduffield/lazygit/pkg/commands/git_commands"
@@ -443,11 +441,7 @@ func (self *CommitDiffActions) patchEndpoints(target *commitDiffTarget) (string,
 // patchBuilderPath turns the absolute path a diff line carries into the repo-relative
 // one the patch builder keys a file by, and "" for a path that is no file of this repo.
 func (self *CommitDiffActions) patchBuilderPath(path string) string {
-	relativePath, err := filepath.Rel(self.c.Git().RepoPaths.WorktreePath(), path)
-	if err != nil || strings.HasPrefix(relativePath, "..") {
-		return ""
-	}
-	return filepath.ToSlash(relativePath)
+	return repoRelativePath(self.c.Git().RepoPaths.WorktreePath(), path)
 }
 
 // indexOfTargetCommit finds the commit the diff belongs to among the commits of the

pkg/gui/controllers/diff_paths.go

@@ -2,6 +2,7 @@ package controllers
 
 import (
 	"path"
+	"path/filepath"
 	"strings"
 
 	"github.com/jesseduffield/lazygit/pkg/gui/filetree"
@@ -130,3 +131,14 @@ func isInDir(path string, dir string) bool {
 	// "." is the root item, which contains every file
 	return dir == "." || strings.HasPrefix(path, dir+"/")
 }
+
+// repoRelativePath turns the absolute path a diff line carries into the one git speaks
+// of the file: relative to the worktree, with forward slashes. It is "" for a path that
+// is no file of this repo, which a diff renderer's own naming of a line can produce.
+func repoRelativePath(worktreePath string, absolutePath string) string {
+	relativePath, err := filepath.Rel(worktreePath, absolutePath)
+	if err != nil || strings.HasPrefix(relativePath, "..") {
+		return ""
+	}
+	return filepath.ToSlash(relativePath)
+}

pkg/gui/controllers/working_tree_diff_actions.go

@@ -2,7 +2,6 @@ package controllers
 
 import (
 	"fmt"
-	"path/filepath"
 	"strings"
 
 	"github.com/jesseduffield/generics/set"
@@ -262,11 +261,11 @@ func (self *WorkingTreeDiffActions) applyDiffLineSelection(
 // fileForDiffLinePath maps the absolute path a diff line carries to the working tree
 // file it belongs to, or nil for a path that is no file of this repo's working tree.
 func (self *WorkingTreeDiffActions) fileForDiffLinePath(path string) *models.File {
-	relativePath, err := filepath.Rel(self.c.Git().RepoPaths.WorktreePath(), path)
-	if err != nil {
+	relativePath := repoRelativePath(self.c.Git().RepoPaths.WorktreePath(), path)
+	if relativePath == "" {
 		return nil
 	}
-	return self.context().FileTreeViewModel.GetFile(filepath.ToSlash(relativePath))
+	return self.context().FileTreeViewModel.GetFile(relativePath)
 }
 
 // applyDiffLines applies the given change lines of one file — a line, a hunk, a range —