prompt: introduce GIT_PS1_STATESEPARATOR

prompt: introduce GIT_PS1_STATESEPARATOR

A typical prompt looks like:

    artagnon|master *=:~/src/git$
                   ^
                   why do we have this space?

Nobody has branch names that end with +, *, =, < or > anyway, so it
doesn't serve the purpose of disambiguation.

Make this separator configurable via GIT_PS1_STATESEPARATOR.  This means
that you can set it to "" and get this prompt:

    artagnon|master*=:~/src/git$

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>