Merge branch 'jh/diff-index-line-abbrev'

* jh/diff-index-line-abbrev:
  diff.c: Ensure "index $from..$to" line contains unambiguous SHA1s

Conflicts:
	diff.c

diff.c

@@ -2430,9 +2430,9 @@ static void fill_metainfo(struct strbuf *msg,
 			    (!fill_mmfile(&mf, two) && diff_filespec_is_binary(two)))
 				abbrev = 40;
 		}
-		strbuf_addf(msg, "%sindex %.*s..%.*s", set,
-			    abbrev, sha1_to_hex(one->sha1),
-			    abbrev, sha1_to_hex(two->sha1));
+		strbuf_addf(msg, "%sindex %s..", set,
+			    find_unique_abbrev(one->sha1, abbrev));
+		strbuf_addstr(msg, find_unique_abbrev(two->sha1, abbrev));
 		if (one->mode == two->mode)
 			strbuf_addf(msg, " %06o", one->mode);
 		strbuf_addf(msg, "%s\n", reset);

t/t4044-diff-index-unique-abbrev.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+test_description='test unique sha1 abbreviation on "index from..to" line'
+. ./test-lib.sh
+
+cat >expect_initial <<EOF
+100644 blob 51d2738463ea4ca66f8691c91e33ce64b7d41bb1	foo
+EOF
+
+cat >expect_update <<EOF
+100644 blob 51d2738efb4ad8a1e40bed839ab8e116f0a15e47	foo
+EOF
+
+test_expect_success 'setup' '
+	echo 4827 > foo &&
+	git add foo &&
+	git commit -m "initial" &&
+	git cat-file -p HEAD: > actual &&
+	test_cmp expect_initial actual &&
+	echo 11742 > foo &&
+	git commit -a -m "update" &&
+	git cat-file -p HEAD: > actual &&
+	test_cmp expect_update actual
+'
+
+cat >expect <<EOF
+index 51d27384..51d2738e 100644
+EOF
+
+test_expect_success 'diff does not produce ambiguous index line' '
+	git diff HEAD^..HEAD | grep index > actual &&
+	test_cmp expect actual
+'
+
+test_done