git p4: temp branch name should use / even on windows

git p4: temp branch name should use / even on windows

Commit fed2369 (git-p4: Search for parent commit on branch creation,
2012-01-25) uses temporary branches to help find the parent of a
new p4 branch.  The temp branches are of the form "git-p4-tmp/%d"
for some p4 change number.  Mistakenly, this string was made
using os.path.join() instead of just string concatenation.  On
windows, this turns into a backslash (\), which is not allowed in
git branch names.

Reported-by: Casey McGinty <casey.mcginty@gmail.com>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>