for-each-ref: introduce %(HEAD) asterisk marker

'git branch' shows which branch you are currently on with an '*', but
'git for-each-ref' misses this feature.  So, extend its format with
%(HEAD) for the same effect.

Now you can use the following format in for-each-ref:

  %(HEAD) %(refname:short)

to display an asterisk next to the current ref.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -93,6 +93,10 @@ upstream::
 	from the displayed ref. Respects `:short` in the same way as
 	`refname` above.
 
+HEAD::
+	'*' if HEAD matches current ref (the checked out branch), ' '
+	otherwise.
+
 In addition to the above, for commit and tag objects, the header
 field names (`tree`, `parent`, `object`, `type`, and `tag`) can
 be used to specify the value in the header field.

builtin/for-each-ref.c

@@ -75,6 +75,7 @@ static struct {
 	{ "upstream" },
 	{ "symref" },
 	{ "flag" },
+	{ "HEAD" },
 };
 
 /*
@@ -675,8 +676,16 @@ static void populate_value(struct refinfo *ref)
 				v->s = xstrdup(buf + 1);
 			}
 			continue;
-		}
+		} else if (!strcmp(name, "HEAD")) {
-		else
+			const char *head;
+			unsigned char sha1[20];
+			head = resolve_ref_unsafe("HEAD", sha1, 1, NULL);
+			if (!strcmp(ref->refname, head))
+				v->s = "*";
+			else
+				v->s = " ";
+			continue;
+		} else
 			continue;
 
 		formatp = strchr(name, ':');

t/t6300-for-each-ref.sh

@@ -77,6 +77,7 @@ test_atom head contents:body ''
 test_atom head contents:signature ''
 test_atom head contents 'Initial
 '
+test_atom head HEAD '*'
 
 test_atom tag refname refs/tags/testtag
 test_atom tag upstream ''
@@ -110,6 +111,7 @@ test_atom tag contents:body ''
 test_atom tag contents:signature ''
 test_atom tag contents 'Tagging at 1151939927
 '
+test_atom tag HEAD ' '
 
 test_expect_success 'Check invalid atoms names are errors' '
 	test_must_fail git for-each-ref --format="%(INVALID)" refs/heads