FS: Allow to manually set the path to the Git system config file

FS: Allow to manually set the path to the Git system config file

Now that d7a4473 removed the gitprefix property, we did not have a way to
specify the path to the Git system config file in case
discoverGitSystemConfig() fails. Fix that by introducing a member variable
that caches the result of discoverGitSystemConfig() as well as a setter
method to overwrite the content of that variable.

Change-Id: Icd965bffbe2f11b18c9505ee2ddd2afad5b64d70
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>