Make `pkg/git` tests not modifying global git config
Make `pkg/git` tests not modifying global git config

Before this patch, running unit tests on `pkg/git` would modify the
content of the machine's global git configuration (usually
`$HOME/.git/config`).

This makes sure we setup a proper isolated git config, per test.

Signed-off-by: Vincent Demeester <b98c0fd48a8945096c766de790fe70420dd2f223@redhat.com>

tektoncd/pipeline,tektoncd/pipeline