virttest: Change working dir before showing git log

When run download_test_provider if the test_provider is already
downloaded, current working dir won't change before running "git log -1".

This will output false log info or won't work at all if cwd is not a git
repo.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #1444 from Hao-Liu/fix_tp_download

virttest: Change working dir before showing git log