fast-import: Fail if a non-existant commit is used for merge

Johannes Sixt noticed during one of his own imports that fast-import
did not fail if a non-existant commit is referenced by SHA-1 value
as an argument to the 'merge' command.  This allowed the user to
unknowingly create commits that would fail in fsck, as the commit
contents would not be completely reachable.

A side effect of this bug was that a frontend process could mark
any SHA-1 object (blob, tree, tag) as a parent of a merge commit.
This should also fail in fsck, as the commit is not a valid commit.

We now use the same rule as the 'from' command.  If a commit is
referenced in the 'merge' command by hex formatted SHA-1 then the
SHA-1 must be a commit or a tag that can be peeled back to a commit,
the commit must already exist, and must be readable by the core Git
infrastructure code.  This requirement means that the commit must
have existed prior to fast-import starting, or the commit must have
been flushed out by a prior 'checkpoint' command.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1752,7 +1752,14 @@ static struct hash_list *cmd_merge(unsigned int *count)
 			if (oe->type != OBJ_COMMIT)
 				die("Mark :%" PRIuMAX " not a commit", idnum);
 			hashcpy(n->sha1, oe->sha1);
-		} else if (get_sha1(from, n->sha1))
+		} else if (!get_sha1(from, n->sha1)) {
+			unsigned long size;
+			char *buf = read_object_with_reference(n->sha1,
+				type_names[OBJ_COMMIT], &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);
 
 		n->next = NULL;