patch.py: Initialize git repo before patching

If PATCHTOOL="git" has been specified but workdir is not git repo
bitbake fails to apply the patches with error message:
Command Error: 'git rev-parse --show-toplevel' exited with 0  Output:
fatal: not a git repository (or any of the parent directories): .git

Fix this by initializing the repo before patching.
This allows binary git patches to be applied.

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>