ref-filter: add short modifier to 'tree' atom

Sometimes while using 'tree' atom, user might want to see abbrev hash
instead of full 40 character hash.

Just like 'objectname', it might be convenient for users to have the
`:short` and `:short=<length>` option for printing 'tree' hash.

Let's introduce `short` option to 'tree' atom.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -222,6 +222,8 @@ worktreepath::
 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.
+Field `tree` can also be used with modifier `:short` and
+`:short=<length>` just like `objectname`.
 
 For commit and tag objects, the special `creatordate` and `creator`
 fields will correspond to the appropriate date or name-email-date tuple

ref-filter.c

@@ -497,7 +497,7 @@ static struct {
 	{ "objectsize", SOURCE_OTHER, FIELD_ULONG, objectsize_atom_parser },
 	{ "objectname", SOURCE_OTHER, FIELD_STR, oid_atom_parser },
 	{ "deltabase", SOURCE_OTHER, FIELD_STR, deltabase_atom_parser },
-	{ "tree", SOURCE_OBJ },
+	{ "tree", SOURCE_OBJ, FIELD_STR, oid_atom_parser },
 	{ "parent", SOURCE_OBJ },
 	{ "numparent", SOURCE_OBJ, FIELD_ULONG },
 	{ "object", SOURCE_OBJ },
@@ -1005,10 +1005,9 @@ static void grab_commit_values(struct atom_value *val, int deref, struct object
 			continue;
 		if (deref)
 			name++;
-		if (!strcmp(name, "tree")) {
-			v->s = xstrdup(oid_to_hex(get_commit_tree_oid(commit)));
-		}
-		else if (!strcmp(name, "numparent")) {
+		if (grab_oid(name, "tree", get_commit_tree_oid(commit), v, &used_atom[i]))
+			continue;
+		if (!strcmp(name, "numparent")) {
 			v->value = commit_list_count(commit->parents);
 			v->s = xstrfmt("%lu", (unsigned long)v->value);
 		}

t/t6300-for-each-ref.sh

@@ -116,6 +116,9 @@ test_atom head objectname:short $(git rev-parse --short refs/heads/master)
 test_atom head objectname:short=1 $(git rev-parse --short=1 refs/heads/master)
 test_atom head objectname:short=10 $(git rev-parse --short=10 refs/heads/master)
 test_atom head tree $(git rev-parse refs/heads/master^{tree})
+test_atom head tree:short $(git rev-parse --short refs/heads/master^{tree})
+test_atom head tree:short=1 $(git rev-parse --short=1 refs/heads/master^{tree})
+test_atom head tree:short=10 $(git rev-parse --short=10 refs/heads/master^{tree})
 test_atom head parent ''
 test_atom head numparent 0
 test_atom head object ''
@@ -167,6 +170,9 @@ test_atom tag objectname:short $(git rev-parse --short refs/tags/testtag)
 test_atom head objectname:short=1 $(git rev-parse --short=1 refs/heads/master)
 test_atom head objectname:short=10 $(git rev-parse --short=10 refs/heads/master)
 test_atom tag tree ''
+test_atom tag tree:short ''
+test_atom tag tree:short=1 ''
+test_atom tag tree:short=10 ''
 test_atom tag parent ''
 test_atom tag numparent ''
 test_atom tag object $(git rev-parse refs/tags/testtag^0)