[Bug 51877] improve POSIX compatibility for wine-staging's patches/gitapply.sh

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=51877

--- Comment #12 from Damjan Jovanovic  ---
(In reply to Zebediah Figura from comment #6)
> Although, frankly, I kind of would rather get rid
> of that script entirely and just require git.

Apparently, that's possible.

All that's needed to get "git apply" working when a parent directory is a Git
repository, is to either:
* Pass the --directory path/to/patch/base/directory option to "git apply"
* Even easier: set the environment variable GIT_DIR=. which will stop it from
searching parent directories for a Git repository.