Hide prompt for ignored present working directory

Optionally hide the git-prompt when the present working directory
is ignored. Behavior is triggered by either (or both) a non-empty
environmental variable, GIT_PS1_HIDE_ON_IGNORED_PWD, or the local
repository config setting bash.hideOnIgnoredPwd. In the absence
of either of these settings this change has no effect. This
change has no effect on directories that are not ignored by git.