Enable custom __GIT_PROMPT_DIR as an env var

I'd like to directly point bash to where I checked out the source code.
With this change I can customize at least the base directory with an environment variable while the default value is still there.
When no upstream is configured, the script behaves as if push.default=current
Adding virtualenv with git prompt
Merge pull request #5 from mrahma01/master

Virtualenv with git prompt
Merge pull request #1 from skuro/patch-1

Enable custom __GIT_PROMPT_DIR as an env var
Merge pull request #2 from pkok/master

When no upstream is configured, the script behaves as if push.default=current
Merge branch 'master' into topic/time24

Not applied two lines convention introduced in
https://github.com/magicmonty/bash-git-prompt/pull/5/files

Conflicts:
	gitprompt.sh