use strbuf_add_unique_abbrev() for adding short hashes, part 2

Call strbuf_add_unique_abbrev() to add abbreviated hashes to strbufs
instead of taking detours through find_unique_abbrev() and its static
buffer.  This is shorter and a bit more efficient.

1eb47f167d65d1d305b9c196a1bb40eb96117cb1 already converted six cases,
this patch covers three more.

A semantic patch for Coccinelle is included for easier checking for
new cases that might be introduced in the future.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index 534c12e..ceaac82 100644
--- a/diff.c
+++ b/diff.c
@@ -3086,7 +3086,7 @@ static void fill_metainfo(struct strbuf *msg,
 		}
 		strbuf_addf(msg, "%s%sindex %s..", line_prefix, set,
 			    find_unique_abbrev(one->oid.hash, abbrev));
-		strbuf_addstr(msg, find_unique_abbrev(two->oid.hash, abbrev));
+		strbuf_add_unique_abbrev(msg, two->oid.hash, abbrev);
 		if (one->mode == two->mode)
 			strbuf_addf(msg, " %06o", one->mode);
 		strbuf_addf(msg, "%s\n", reset);