branch: factor out "upstream is not a branch" error messages

This message is duplicated, and is quite long. Let's factor
it out, which avoids the repetition and the long lines. It
will also make future patches easier as we tweak the
message.

While we're at it, let's also mark it for translation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -197,6 +197,9 @@ int validate_new_branchname(const char *name, struct strbuf *ref,
 	return 1;
 }
 
+static const char upstream_not_branch[] =
+N_("Cannot setup tracking information; starting point is not a branch.");
+
 void create_branch(const char *head,
 		   const char *name, const char *start_name,
 		   int force, int reflog, int clobber_head,
@@ -231,14 +234,14 @@ void create_branch(const char *head,
 	case 0:
 		/* Not branching from any existing branch */
 		if (explicit_tracking)
-			die("Cannot setup tracking information; starting point is not a branch.");
+			die(_(upstream_not_branch));
 		break;
 	case 1:
 		/* Unique completion -- good, only if it is a real branch */
 		if (prefixcmp(real_ref, "refs/heads/") &&
 		    prefixcmp(real_ref, "refs/remotes/")) {
 			if (explicit_tracking)
-				die("Cannot setup tracking information; starting point is not a branch.");
+				die(_(upstream_not_branch));
 			else
 				real_ref = NULL;
 		}