diff: munmap() file contents before running external diff

diff: munmap() file contents before running external diff

When running an external diff from, say, a diff tool, it is safe to
assume that we want to write the files in question. On Windows, that
means that there cannot be any other process holding an open handle to
said files, or even just a mapped region.

So let's make sure that `git diff` itself is not holding any open handle
to the files in question.

In fact, we will just release the file pair right away, as the external
diff uses the files we just wrote, so we do not need to hold the file
contents in memory anymore.

This fixes https://github.com/git-for-windows/git/issues/1315

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