This avoids the following misleading error message:
error: unable to create temporary sha1 filename ./objects/15: File exists
mkstemp can fail for many reasons, one of which, ENOENT, can occur if the directory for the temp file doesn't exist. create_tmpfile tried to handle this case by always trying to mkdir the directory, even if it already existed. This caused errno to be clobbered, so one cannot tell why mkstemp really failed, and it truncated the buffer to just the directory name, resulting in the strange error message shown above.
Note that in both occasions that I've seen this failure, it has not been due to a missing directory, or bad permissions, but some other, unknown mkstemp failure mode that did not occur when I ran git again. This code could perhaps be made more robust by retrying mkstemp, in case it was a transient failure.
Signed-off-by: Joey Hess <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>