git client fails if core.bare is unset

Sometimes git repositories are configured without core.bare being set (using
git repo tool for example, or using msysgit on windows). The current rbtools
git client implementation breaks in this situation.

Additionally, core.bare can be either "1" or "true". This change adds "1" to
the list of values checked.

Testing Done:
post successfully this draft review
* with core.bare = false from .git/config
* without core.bare = false from .git/config

Bugs closed: 3213

Reviewed at https://reviews.reviewboard.org/r/5342/