t / t4101 / diff.1-3on commit sha1_file: link() returns -1 on failure, not errno (e32c0a9)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,3 @@
   4 a
   5-b
   6 c
   7+b
   8\ No newline at end of file