username/password case.
For cache to store change:
* By default, cache only last 900 seconds, gitsync will break after
that. See https://git-scm.com/docs/git-credential-cache.
* The test won't work with cache since the test don't have access to
the default unix socket location; XDG_CACHE_HOME override also can
pre-create a socket in advance.
* `store` put the credential into a file, much easier to debug than cache.
* Considering anyone have access to the pod already able to get the
credential via environment variables or yaml configs, so put it in
file won't make it less secure.
For the new password test:
1. askpass_git.sh provided to simulate a git with password challenge.
2. Need and only need to similate "clone" action, need to bypass other
actions like config/credential setup.
3. See `credential fill` is the official git action to ask password,
see https://git-scm.com/docs/git-credential.
This change resolved issue #196.
Chuanying Du