[PATCH v2 2/3] fast-import: allow "merge $null_sha1" command

[Dmitry Ivankov <divanorama@gmail.com>]

"from $null_sha1" and "merge $empty_branch" are already allowed so
allow "merge $null_sha1" command too.

However such 'merge' has no effect on the import. It's made allowed
just to unify null_sha1 commits handling a little bit.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c          |   14 ++++++++------
 t/t9300-fast-import.sh |    1 +
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index 419e435..f03da1e 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2631,12 +2631,14 @@ static struct hash_list *parse_merge(unsigned int *count)
 				die("Mark :%" PRIuMAX " not a commit", idnum);
 			hashcpy(n->sha1, oe->idx.sha1);
 		} else if (!get_sha1(from, n->sha1)) {
-			unsigned long size;
-			char *buf = read_object_with_reference(n->sha1,
-				commit_type, &size, n->sha1);
-			if (!buf || size < 46)
-				die("Not a valid commit: %s", from);
-			free(buf);
+			if (!is_null_sha1(n->sha1)) {
+				unsigned long size;
+				char *buf = read_object_with_reference(n->sha1,
+					commit_type, &size, n->sha1);
+				if (!buf || size < 46)
+					die("Not a valid commit: %s", from);
+				free(buf);
+			}
 		} else
 			die("Invalid ref name or SHA1 expression: %s", from);
 
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 5716420..6f4c988 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -864,6 +864,7 @@ Merge J3, J4 into fresh J5.
 COMMIT
 merge refs/heads/J3
 merge refs/heads/J4
+merge 0000000000000000000000000000000000000000
 
 INPUT_END
 test_expect_success \
-- 
1.7.3.4