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.

Originally by Tamir Duberstein but updated by iant & rsc to add
the filepath.Join logic.

Fixes #11211 (again).

Change-Id: I6d29b5ae39ba456088ae1fb5d41014cb91c86897
Reviewed-on: https://go-review.googlesource.com/18323Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/build.go

@@ -322,18 +322,15 @@ 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"))
+	if !filepath.IsAbs(gitDir) {
+		gitDir = filepath.Join(goroot, gitDir)
+	}
+	fi, err := os.Stat(gitDir)
+	return err == nil && fi.IsDir()
 }
 
 /*