Prevent shell expansion while showing repository dirty status in git-prompt.
Improve rebase detection in git-prompt.

Detect either rebase-merge and/or rebase-apply. Sometimes only one or
the other file was found during a pause in rebase.

Rebase-merge only appeared during conflict resolution, but stopping to
edit where there are no conflicts would cause only rebase-apply to
appear.

We want to detect rebase in either scenario.
Improve variable naming in git-prompt.