[PATCH] Fix mkpath abuse in dwim_ref/sha1_name.c

Otherwise the function sometimes fail to resolve obviously correct refnames,
because the string data pointed to by "ref" argument were reused.

Signed-off-by: Alex Riesen <redacted>
---
 sha1_name.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

Frankly, I'm having hard time trying to justify _any_ use of mkpath.
It is a bug waiting to happen every time is any code between the
call site and assignment but strdup.