clone --dissociate: avoid locking pack files

clone --dissociate: avoid locking pack files

When `git clone` is asked to dissociate the repository from the
reference repository whose objects were used, it is quite possible that
the pack files need to be repacked. In that case, the pack files need to
be deleted that were originally hard-links to the reference repository's
pack files.

On platforms where a file cannot be deleted if another process still
holds a handle on it, we therefore need to take pains to release all
pack files and indexes before dissociating.

This fixes https://github.com/git-for-windows/git/issues/446

The test case to demonstrate the breakage technically does not need to
be run on Linux or MacOSX. It won't hurt, either, though.

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