[PATCH v2 1/3] fast-import: do not write null_sha1 as a merge parent

[Dmitry Ivankov <divanorama@gmail.com>]

null_sha1 is used in fast-import to indicate "empty" branches and
should never be actually written out as a commit parent. 'merge'
command lacks is_null_sha1 checks and must be fixed.

It looks like using null_sha1 or empty branches in 'from' command
is legal and/or an intended option (it has been here from the very
beginning and survived). So leave it allowed for 'merge' command too,
and just like with 'from' command silently skip null_sha1 parents.

Add a simple test for null_sha1 merge parents.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c          |    3 ++-
 t/t9300-fast-import.sh |   21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index eed97c8..419e435 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2734,7 +2734,8 @@ static void parse_new_commit(void)
 		strbuf_addf(&new_data, "parent %s\n", sha1_to_hex(b->sha1));
 	while (merge_list) {
 		struct hash_list *next = merge_list->next;
-		strbuf_addf(&new_data, "parent %s\n", sha1_to_hex(merge_list->sha1));
+		if (!is_null_sha1(merge_list->sha1))
+			strbuf_addf(&new_data, "parent %s\n", sha1_to_hex(merge_list->sha1));
 		free(merge_list);
 		merge_list = next;
 	}
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index c17f52e..5716420 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -850,6 +850,27 @@ INPUT_END
 test_expect_success \
 	'J: tag must fail on empty branch' \
 	'test_must_fail git fast-import <input'
+
+cat >input <<INPUT_END
+reset refs/heads/J3
+
+reset refs/heads/J4
+from 0000000000000000000000000000000000000000
+
+commit refs/heads/J5
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+Merge J3, J4 into fresh J5.
+COMMIT
+merge refs/heads/J3
+merge refs/heads/J4
+
+INPUT_END
+test_expect_success \
+	'J: allow merge with empty branch' \
+	'git fast-import <input &&
+	git rev-parse --verify J5 &&
+	test_must_fail git rev-parse --verify J5^'
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-- 
1.7.3.4