vpm: detect unpushed commits on detached HEAD too

`git rev-list --branches --not --remotes` only walks local branch refs,
so it misses local commits made on a detached HEAD — which is exactly
the state vpm leaves a versioned install in (`git clone -b <tag>`).
Add `HEAD` to the rev-list so a clean working tree with a local commit
on a detached HEAD is also detected.

Author: medvednikov

cmd/tools/vpm/install.v

@@ -182,7 +182,10 @@ fn has_local_git_changes(path string) bool {
 	if status.output.trim_space() != '' {
 		return true
 	}
-	unpushed := os.execute_opt('git -C ${quoted} rev-list --branches --not --remotes') or {
+	// Include `HEAD` so commits made on a detached HEAD (e.g. after
+	// `git clone -b <tag>`, the layout vpm uses for versioned installs) are
+	// also detected. `--branches` alone only walks local branch refs.
+	unpushed := os.execute_opt('git -C ${quoted} rev-list HEAD --branches --not --remotes') or {
 		return false
 	}
 	return unpushed.output.trim_space() != ''