format-patch: create leading components of output directory

format-patch: create leading components of output directory

'git format-patch -o <outdir>' did an equivalent of 'mkdir <outdir>'
not 'mkdir -p <outdir>', which is being corrected.

Avoid the usage of 'adjust_shared_perm' on the leading directories which
may have security implications. Achieved by temporarily disabling of
'config.sharedRepository' like 'git init' does.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>