use skip_prefix() to avoid more magic numbers

Continue where ae021d87 (use skip_prefix to avoid magic numbers) left off
and use skip_prefix() in more places for determining the lengths of prefix
strings to avoid using dependent constants and other indirect methods.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index b0fe784..cff7802 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1294,6 +1294,7 @@
 static int git_status_config(const char *k, const char *v, void *cb)
 {
 	struct wt_status *s = cb;
+	const char *slot_name;
 
 	if (starts_with(k, "column."))
 		return git_column_config(k, v, "status", &s->colopts);
@@ -1323,8 +1324,9 @@
 		s->display_comment_prefix = git_config_bool(k, v);
 		return 0;
 	}
-	if (starts_with(k, "status.color.") || starts_with(k, "color.status.")) {
-		int slot = parse_status_slot(k, 13);
+	if (skip_prefix(k, "status.color.", &slot_name) ||
+	    skip_prefix(k, "color.status.", &slot_name)) {
+		int slot = parse_status_slot(k, slot_name - k);
 		if (slot < 0)
 			return 0;
 		if (!v)
@@ -1513,13 +1515,11 @@
 	diff_setup_done(&rev.diffopt);
 
 	head = resolve_ref_unsafe("HEAD", junk_sha1, 0, NULL);
-	printf("[%s%s ",
-		starts_with(head, "refs/heads/") ?
-			head + 11 :
-			!strcmp(head, "HEAD") ?
-				_("detached HEAD") :
-				head,
-		initial_commit ? _(" (root-commit)") : "");
+	if (!strcmp(head, "HEAD"))
+		head = _("detached HEAD");
+	else
+		skip_prefix(head, "refs/heads/", &head);
+	printf("[%s%s ", head, initial_commit ? _(" (root-commit)") : "");
 
 	if (!log_tree_commit(&rev, commit)) {
 		rev.always_show_header = 1;