for-each-ref: Field with abbreviated objectname

Introduce a :short modifier to objectname which outputs the abbreviated
object name.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -86,6 +86,7 @@ objectsize::
 
 objectname::
 	The object name (aka SHA-1).
+	For a non-ambiguous abbreviation of the object name append `:short`.
 
 upstream::
 	The name of a local ref which can be considered ``upstream''

builtin/for-each-ref.c

@@ -227,6 +227,9 @@ static void grab_common_values(struct atom_value *val, int deref, struct object
 			strcpy(s, sha1_to_hex(obj->sha1));
 			v->s = s;
 		}
+		else if (!strcmp(name, "objectname:short")) {
+			v->s = find_unique_abbrev(obj->sha1, DEFAULT_ABBREV);
+		}
 	}
 }
 

t/t6300-for-each-ref.sh

@@ -295,6 +295,15 @@ test_expect_success 'Check short upstream format' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+67a36f1
+EOF
+
+test_expect_success 'Check short objectname format' '
+	git for-each-ref --format="%(objectname:short)" refs/heads >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'Check for invalid refname format' '
 	test_must_fail git for-each-ref --format="%(refname:INVALID)"
 '