sha1_file: link() returns -1 on failure, not errno

5723fe7 (Avoid cross-directory renames and linking on object creation,
2008-06-14) changed the call to use link() directly instead of through a
custom wrapper, but forgot that it returns 0 or -1, not 0 or errno.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2136,7 +2136,9 @@ static void write_sha1_file_prepare(const void *buf, unsigned long len,
  */
 int move_temp_to_file(const char *tmpfile, const char *filename)
 {
-	int ret = link(tmpfile, filename);
+	int ret = 0;
+	if (link(tmpfile, filename))
+		ret = errno;
 
 	/*
 	 * Coda hack - coda doesn't like cross-directory links,