cmd/dist: improve isGitRepo to handle git "worktree"s

Simply checking the exit code of `git rev-parse --git-dir` should
suffice here, but that requires deviating from the infrastructure
provided by `run`, so I've left that for a future change.

Fixes #11211.

Change-Id: I7cbad86a8a06578f52f66f734f5447b597ddc962
Reviewed-on: https://go-review.googlesource.com/18213Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/build.go

@@ -322,18 +322,12 @@ func findgoversion() string {
 
 // isGitRepo reports whether the working directory is inside a Git repository.
 func isGitRepo() bool {
-	p := ".git"
-	for {
-		fi, err := os.Stat(p)
-		if os.IsNotExist(err) {
-			p = filepath.Join("..", p)
-			continue
-		}
-		if err != nil || !fi.IsDir() {
-			return false
-		}
-		return true
-	}
+	// NB: simply checking the exit code of `git rev-parse --git-dir` would
+	// suffice here, but that requires deviating from the infrastructure
+	// provided by `run`.
+	gitDir := chomp(run(goroot, 0, "git", "rev-parse", "--git-dir"))
+	fi, err := os.Stat(gitDir)
+	return err == nil && fi.IsDir()
 }
 
 /*