debian: package git-prompt script (get __git_ps1 working again)

debian: package git-prompt script (get __git_ps1 working again)

Danny Yates writes:

> I've recently upgraded git on an Oneiric server to
> 1:1.8.0-0ppa1~oneiric1, and seem to have lost the __git_ps1 function. It
> used to be in /etc/bash_completion.d/git.

__git_ps1 has been split out from the main completion script into a
separate git-prompt.sh library (see v1.7.12-rc0~67^2~2, 2012-05-22)
and unfortunately there is no standard place to install it.

This patch takes the following approach:

 * install contrib/completion/git-prompt.sh to
   $(gitexecdir)/git-sh-prompt, which based on upstream discussion
   is at least a possible place for that script.

 * source git-sh-prompt in /etc/bash_completion.d/git-prompt so
   existing configurations that relied on __git_ps1 becoming available
   when bash-completion is loaded continue to work without fuss.

 * no NEWS entry yet.  We can add one encouraging users to update
   their configuration once the filename for the git-prompt scriptlet
   has been decided.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>