tests: add tests for the bash prompt functions in the completion script

The tests cover the discovery of the '.git' directory in the
__gitdir() function in different scenarios, and the prompt itself,
i.e. branch name, detached heads, operations (rebase, merge,
cherry-pick, bisect), and status indicators (dirty, stash, untracked
files; but not the upstream status).

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>