gitk: escape file paths before piping to git log

gitk: escape file paths before piping to git log

We just started piping the file paths via `stdin` instead of passing
them via the command-line, to avoid running into command-line
limitations.

However, since we now pipe the file paths, we need to take care of
special characters.

This fixes https://github.com/git-for-windows/git/issues/2293

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