Setting file timestamps to commit time (git-checkout)

Me and some colleagues work on gcc in lots of different branches.
For each branch there is a separate build directory for each
branch, e.g. build-a, build-b and build-c.  Let's assume that all
branches are identical at the moment.  If a file in branch a is
changed that triggers a complete rebuild of gcc (e.g.
<target>.opt), rebuilding in build-a takes about an hour.  Now,
 when I switch to one of the other branches, said file is not
identical anymore and stamped with the _current_ time during
checkout.  Although branch b and c have not changed at all, they
will now be rebuilt completely because the timestamp on that files
has changed.  I.e. a chance on one branch forces a rebuild on n
other branches, which can take many hours.

I think this situation could be improved with an option to git-checkout with the following logic:

$ git checkout <new branch>
  FOR EACH <file> in working directory of <new branch>
    IF <file> is identical to the version in the <old branch>
      THEN leave the file untouched
    ELSE IF <commit timestamp> of the HEAD of the <new branch>
            is in the future
      THEN checkout the new version of <file> and stamp it with
           the current time
    ELSE (commit timestamp is current or in the past)
      THEN checkout the new version of <file> and stamp it with
           the commit timestamp of the current HEAD of <new branch>

Any comments?  Is there already a way to do this?

(Please do not cc me on replies, I'm subscribed to the list.)

Ciao

Dominik ^_^  ^_^