Fix repository corruption when using marks for modified blobs.

Apparently we did not copy the blob SHA1 into the stack variable
'sha1' when a mark is used to refer to a prior blob.  This code
was not previously tested as the Mozilla CVS -> git-fast-import
program always fed us full SHA1s for modified blobs and did not
use the mark feature there.

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

fast-import.c

@@ -1378,6 +1378,7 @@ static void file_change_m(struct branch *b)
 	if (*p == ':') {
 		char *x;
 		oe = find_mark(strtoul(p + 1, &x, 10));
+		hashcpy(sha1, oe->sha1);
 		p = x;
 	} else {
 		if (get_sha1_hex(p, sha1))