git-generate: fix file tree walking to work with git worktrees

Currently git-generate snapshots the git directory before it makes
changes so it can identify newly-created files.

The file tree walking it performs explicitly ignores the .git directory,
but sometimes the .git directory is actually a file, like when git
worktrees are in use. In this case, the file tree walking code will
actually return fs.SkipDir through the callback passed to
filepath.WalkDir and will skip iterating over the rest of directories in
the repository.

Fix this by only returning fs.SkipDir only when .git is a directory.

git-generate/main.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.16
 // +build go1.16
 
 // Git-generate regenerates a commit from a script kept in the commit message.
@@ -221,7 +222,14 @@ func gitDir(dir string, args ...string) string {
 func walkGit(dir string, f func(path string)) {
 	filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
 		if d.Name() == ".git" {
-			return fs.SkipDir
+			// .git is usually a directory, in which case we want to skip it.
+			// But in some cases it may be a file (git worktrees), so we need
+			// to be careful not to return fs.SkipDir or else we'll skip .git's
+			// parent, which is usually the repository root.
+			if d.IsDir() {
+				return fs.SkipDir
+			}
+			return nil
 		}
 		if d.IsDir() {
 			return nil