Added the envvar GIT_PS1_SHOWUNTRACKEDFILES to 'git-completion.bash'.
When set to a nonempty value, then the char '%' will be shown next
to the branch name in the bash prompt.
Signed-off-by: Daniel Trstenjak <daniel.trstenjak@science-computing.de> Acked-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>