cvsexportcommit: introduce -W for shared working trees (between Git and CVS)

cvsexportcommit: introduce -W for shared working trees (between Git and CVS)

If you have a CVS checkout, it is easy to import the CVS history by
calling "git cvsimport".  However, interacting with the CVS repository
using "git cvsexportcommit" was cumbersome, since that script assumes
separate working directories for Git and CVS.

Now, you can call cvsexportcommit with the -W option.  This will
automatically discover the GIT_DIR, and it will check out the parent
commit before exporting the commit.

The intended workflow is this:

$ CVSROOT=$URL cvs co module
$ cd module
$ git cvsimport
hack, hack, hack, making two commits, cleaning them up using rebase -i.
$ git cvsexportcommit -W -c -p -u HEAD^
$ git cvsexportcommit -W -c -p -u HEAD

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