show-branch --current: do not barf on detached HEAD

The code assumed that there always is the current branch, but the result
from resolve_ref() on detached HEAD does not even start with "refs/heads/".

Originally noticed and fixed by Stephan Beyer.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-show-branch.c

@@ -782,8 +782,8 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 				has_head++;
 		}
 		if (!has_head) {
-			int pfxlen = strlen("refs/heads/");
-			append_one_rev(head + pfxlen);
+			int offset = !prefixcmp(head, "refs/heads/") ? 11 : 0;
+			append_one_rev(head + offset);
 		}
 	}