checkout: abbreviate hash in suggest_reattach

After printing the list of left-behind commits (with abbreviated
hashes), use an abbreviated hash in the suggested 'git branch' command;
there's no point in outputting a full 40-character hex string in some
friendly advice.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -729,7 +729,7 @@ static void suggest_reattach(struct commit *commit, struct rev_info *revs)
 			"If you want to keep them by creating a new branch, "
 			"this may be a good time\nto do so with:\n\n"
 			" git branch new_branch_name %s\n\n"),
-			sha1_to_hex(commit->object.sha1));
+			find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
 }
 
 /*