Switch GIT to use a meta-pkg hierarchy like, for example, devel/subversion.

Switch GIT to use a meta-pkg hierarchy like, for example, devel/subversion.
Also, update to 1.5.3.7.

Fixes since v1.5.3.6
--------------------

 * git-send-email added 8-bit contents to the payload without
   marking it as 8-bit in a CTE header.

 * "git-bundle create a.bndl HEAD" dereferenced the symref and
   did not record the ref as 'HEAD'; this prevented a bundle
   from being used as a normal source of git-clone.

 * The code to reject nonsense command line of the form
   "git-commit -a paths..." and "git-commit --interactive
   paths..." were broken.

 * Adding a signature that is not ASCII-only to an original
   commit that is ASCII-only would make the result non-ASCII.
   "git-format-patch -s" did not mark such a message correctly
   with MIME encoding header.

 * git-add sometimes did not mark the resulting index entry
   stat-clean.  This affected only cases when adding the
   contents with the same length as the previously staged
   contents, and the previous staging made the index entry
   "racily clean".

 * git-commit did not honor GIT_INDEX_FILE the user had in the
   environment.

 * When checking out a revision, git-checkout did not report where the
   updated HEAD is if you happened to have a file called HEAD in the
   work tree.

 * "git-rev-list --objects" mishandled a tree that points at a
   submodule.

 * "git cvsimport" was not ready for packed refs that "git gc" can
   produce and gave incorrect results.

 * Many scripted Porcelains were confused when you happened to have a
   file called "HEAD" in your work tree.

 * Miscellaneous updates to the user manual and documentation.