vcs: Do not interactively ask for a password (for git).

Fixes potential "Username for 'https://github.com': " interactive
prompts for private repos.
Thank you @pornel for the solution from bower/bower#1009. It took me
half an hour to find a working solution for this problem, which is odd
given that ls-remote is supposed to be a low-level plumbing command to
be used primarily by scripts. :/

git.go

@@ -96,7 +96,7 @@ func CheckGitRepoLocal(path, branch string) string {
 }
 
 func CheckGitRepoRemote(path, branch string) string {
-	cmd := exec.Command("git", "ls-remote", "--heads", "origin", branch)
+	cmd := exec.Command("git", "-c", "core.askpass=true", "ls-remote", "--heads", "origin", branch)
 	cmd.Dir = path
 
 	if out, err := cmd.Output(); err == nil && len(out) >= gitRevisionLength {