completion: Better __git_ps1 support when not in working directory

completion: Better __git_ps1 support when not in working directory

If .git/HEAD is not readable, __git_ps1 does nothing.

If --is-in-git-dir, __git_ps1 returns " (GIT_DIR!)" as a cautionary
note. The previous behavior would show the branch name (and would
optionally attempt to determine the dirtyState of the directory, which
was impossible because a "git diff" was used).

If --is-in-work-tree, __git_ps1 returns the branch name. Additionally,
if showDirtyState is on, the dirty state is displayed.

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>