tests: use 'git -C' instead of '--git-dir'
tests: use 'git -C' instead of '--git-dir'

Using --git-dir will only place git objects into this directory. It will
continue to use the current directory as working tree for all git
operations. This means that added paths were relative to the execution
of the test script, i.e. all paths had the test-* repository path as
prefix.

trac-hacks/trac-github