rerere: make rr-cache fanout directory honor umask
rerere: make rr-cache fanout directory honor umask

This is the last remaining call to mkdir(2) that restricts the permission
bits by passing 0755.  Just use the same mkdir_in_gitdir() used to create
the leaf directories.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git