bash prompt: use bash builtins to check stash state

bash prompt: use bash builtins to check stash state

When the environment variable $GIT_PS1_SHOWSTASHSTATE is set
__git_ps1() checks the presence of stashes by running 'git rev-parse
--verify refs/stash'.  This command not only checks that the
'refs/stash' ref exists but also, well, verifies that it's a valid
ref.

However, we don't need to be that thorough for the bash prompt.  We
can omit that verification and only check whether 'refs/stash' exists
or not.  Since 'git pack-refs' never packs 'refs/stash', it's a matter
of checking the existence of a ref file.  Perform this check using
only bash builtins to spare the overhead of fork()+exec()ing a git
process.

Also run 'git pack-refs --all' in the corresponding test to document
that the prompt script depends on 'git pack-refs' not packing
'refs/stash' and to catch possible breakages should this behavior ever
change.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>