git-cvsserver: add mechanism for managing working tree and current di…

…rectory

There are various reasons git-cvsserver needs to manipulate the current
directory, and this patch attempts to clarify and validate such changes:

1. Temporary empty working directory (with index) for certain operations
   that require an index file to work.
2. Use a temporary directory with temporary file names for doing
   merges of user's dirty sandbox state with latest changes in
   repository.
3. Coming up soon: Set up an index and either a valid or empty
   working directory when calling git-check-attr to decide
   if a file should be marked binary (-kb).

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>