fast-import: add support to delete refs

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -483,6 +483,9 @@ Marks must be declared (via `mark`) before they can be used.
 * Any valid Git SHA-1 expression that resolves to a commit.  See
   ``SPECIFYING REVISIONS'' in linkgit:gitrevisions[7] for details.
 
+* The special null SHA-1 (40 zeros) specifies that the branch is to be
+  removed.
+
 The special case of restarting an incremental import from the
 current branch value should be written as:
 ----

fast-import.c

@@ -248,6 +248,7 @@ struct branch {
 	uintmax_t last_commit;
 	uintmax_t num_notes;
 	unsigned active : 1;
+	unsigned delete : 1;
 	unsigned pack_id : PACK_ID_BITS;
 	unsigned char sha1[20];
 };
@@ -1681,10 +1682,13 @@ static int update_branch(struct branch *b)
 	struct ref_lock *lock;
 	unsigned char old_sha1[20];
 
-	if (is_null_sha1(b->sha1))
-		return 0;
 	if (read_ref(b->name, old_sha1))
 		hashclr(old_sha1);
+	if (is_null_sha1(b->sha1)) {
+		if (b->delete)
+			delete_ref(b->name, old_sha1, 0);
+		return 0;
+	}
 	lock = lock_any_ref_for_update(b->name, old_sha1, 0, NULL);
 	if (!lock)
 		return error("Unable to lock %s", b->name);
@@ -2611,8 +2615,11 @@ static int parse_from(struct branch *b)
 			free(buf);
 		} else
 			parse_from_existing(b);
-	} else if (!get_sha1(from, b->sha1))
+	} else if (!get_sha1(from, b->sha1)) {
 		parse_from_existing(b);
+		if (is_null_sha1(b->sha1))
+			b->delete = 1;
+	}
 	else
 		die("Invalid ref name or SHA1 expression: %s", from);
 

t/t9300-fast-import.sh

@@ -2999,4 +2999,22 @@ test_expect_success 'T: ls root tree' '
 	test_cmp expect actual
 '
 
+test_expect_success 'T: delete branch' '
+	git branch to-delete &&
+	git fast-import <<-EOF &&
+	reset refs/heads/to-delete
+	from 0000000000000000000000000000000000000000
+	EOF
+	test_must_fail git rev-parse --verify refs/heads/to-delete
+'
+
+test_expect_success 'T: empty reset doesnt delete branch' '
+	git branch not-to-delete &&
+	git fast-import <<-EOF &&
+	reset refs/heads/not-to-delete
+	EOF
+	git show-ref &&
+	git rev-parse --verify refs/heads/not-to-delete
+'
+
 test_done