Fix for normalization of foreign idents

Hi.

The new behaviour that $Id$ tags containing expanded idents from other version control systems, nice though it is, has a rather serious problem. This is because convert_to_git is no longer the inverse operation of convert_to_working_tree. For native git idents, the transformations are

  $Id$ --(c_t_w_t)--> $Id: 123...$ --(c_t_g)--> $Id$

but for foreign idents, it becomes

  $Id: blah$ --(c_t_w_t)--> $Id: blah$ --(c_t_g)--> $Id$

The result of this is that git may consider even newly checked out files as modified, even though neither the file contents nor its attributes have been modified after the checkout. I say _may_, because it can also happen that it decides based on the time stamps that it doesn't need to compare the actual contents, in which case the file does not show as modified.

The following patch fixes this by preserving the foreign ident also in convert_to_git, meaning that convert_to_git is again the inverse operation of convert_to_working_tree, with the following transformation series:

  $Id: blah$ --(c_t_w_t)--> $Id: blah$ --(c_t_g)--> $Id: blah$

This restores correct and deterministic operation of status and diff, meaning that if the file hasn't actually been modified, no modifications are shown.

As you might suggest, always keeping the foreign ident would mean it is never updated when you commit new versions of the file, which isn't really what we want. Keeping the foreign ident as long as the last modification to the file was made in the previous version control system makes perfect sense, but once we make a commit to the file within git, it should be replaced with a git ident. The patch is therefore slightly more complex, adding an extra parameter to control whether foreign idents are collapsed or not. This parameter is set to true only in the case when index_mem is called with write_object set to true, which is to say when we create a new blob from the working tree (i.e. we are committing the file).

I hope we can agree that this is a sound and unintrusive way of handling the problem. :-)

Incidentally, should one want to create a commit to replace a foreign ident with a native git one without making any other changes to the file, this is still simple to do. All that is needed is to change any character inside the expanded ident in the working tree, and the file will show as modified, and will have the foreign ident removed on commit.

  // Marcus