Improve bash prompt to detect various states like an unfinished merge

Improve bash prompt to detect various states like an unfinished merge

This patch makes the git prompt (when enabled) show if a merge or a
rebase is unfinished. It also detects if a bisect is being done as
well as detached checkouts.

An uncompleted git-am cannot be distinguised from a rebase (the
non-interactive version). Instead of having an even longer prompt
we simply ignore that and hope the power users that use git-am knows
the difference.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>