[tests] Use mocked git user/system configs for repository tests
GitRepositoriesViewBranchHandlingTest.testBranchConfiguration()
failed locally for me because I have pull.rebase=true and
brach.autosetuprebase=always set in my user config. The test,
however, expects those not to be set and tests for
RebaseMode.NONE.
Also let's pretend that no git environment variables were set.
Tests should never use real git configs, and should not depend on
external environment variables.
Change-Id: I897a38c420df37dc01ffde8114d1dac7ad77a3e2
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>