Add current branch in PS1 support to git-completion.bash.

Add current branch in PS1 support to git-completion.bash.

Many users want to display the current branch name of the current git
repository as part of their PS1 prompt, much as their PS1 prompt might
also display the current working directory name.

We don't force our own PS1 onto the user.  Instead we let them craft
their own PS1 string and offer them the function __git_ps1 which they
can invoke to obtain either "" (when not in a git repository) or
"(%s)" where %s is the name of the current branch, as read from HEAD,
with the leading refs/heads/ removed.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>