cmd/dist: show friendlier error message when building outside a Git repo

Fixes #9932

Change-Id: I7943470a1784278a5c6e99c3b66c59d4953734ba
Reviewed-on: https://go-review.googlesource.com/5340Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/build.go

@@ -296,6 +296,11 @@ func findgoversion() string {
 		return chomp(readfile(path))
 	}
 
+	// Show a nicer error message if this isn't a Git repo.
+	if !isGitRepo() {
+		fatal("FAILED: not a Git repo; must put a VERSION file in $GOROOT")
+	}
+
 	// Otherwise, use Git.
 	// What is the current branch?
 	branch := chomp(run(goroot, CheckExit, "git", "rev-parse", "--abbrev-ref", "HEAD"))
@@ -321,6 +326,22 @@ func findgoversion() string {
 	return tag
 }
 
+// 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
+	}
+}
+
 /*
  * Initial tree setup.
  */