completion: split __git_ps1 into a separate script

bash-completion 1.90 shipped with support to load completions
dynamically[1], which means the git completion script wouldn't be loaded
until the user types 'git <tab>'--this creates a problem to people using
__git_ps1(); that function won't be available when the shell is first
created.

For now distributions have workarounded this issue by moving the git
completion to the "compatdir"[2]; this of course is not ideal.

The solution, proposed by Kerrick Staley[3], is to split the git script
in two; the part that deals with __git_ps1() in one (i.e.
git-prompt.sh), and everything else in another (i.e.
git-completion.bash).

Another benefit of this is that zsh user that are not interested in the
bash completion can use it for their prompts, which has been tried
before[4].

The only slight issue is that __gitdir() would be duplicated, but this
is probably not a big deal.

So let's go ahead and move __git_ps1() to a new file.

While at this, I took the liberty to reformat the help text in the new
file.

 [1] http://anonscm.debian.org/gitweb/?p=bash-completion/bash-completion.git;a=commitdiff;h=99c4f7f25f50a7cb2fce86055bddfe389effa559
 [2] http://projects.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/git&id=974380fabb8f9f412990b17063bf578d98c44a82
 [3] http://mid.gmane.org/CANaWP3w9KDu57aHquRRYt8td_haSWTBKs7zUHy-xu0B61gmr9A@mail.gmail.com
 [4] http://mid.gmane.org/1303824288-15591-1-git-send-email-mstormo@gmail.com

Cc: Kerrick Staley <mail@kerrickstaley.com>
Cc: Marius Storm-Olsen <mstormo@gmail.com>
Cc: Ville Skyttä <ville.skytta@iki.fi>
Cc: Dan McGee <dan@archlinux.org>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9903-bash-prompt.sh

@@ -7,7 +7,7 @@ test_description='test git-specific bash prompt functions'
 
 . ./lib-bash.sh
 
-. "$GIT_BUILD_DIR/contrib/completion/git-completion.bash"
+. "$GIT_BUILD_DIR/contrib/completion/git-prompt.sh"
 
 actual="$TRASH_DIRECTORY/actual"