git status: show relative paths when run in a subdirectory

git status: show relative paths when run in a subdirectory

To show the relative paths, the function formerly called quote_crlf()
(now called quote_path()) takes the prefix as an additional argument.

While at it, the static buffers were replaced by strbufs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>