git-mergetool--lib: Make vimdiff retain the current directory

git-mergetool--lib: Make vimdiff retain the current directory

When using difftool with vimdiff it can be unexpected that
the current directory changes to the root of the project.
Tell vim to chdir to the value of $GIT_PREFIX to fix this.

Care is taken to quote the variable so that vim expands it.
This avoids problems when directory names contain spaces.

Signed-off-by: David Aguilar <davvid@gmail.com>
Reported-by: Frédéric Heitzmann <frederic.heitzmann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>