Makefile: improve repo creation target, avoid := notation

The := notation is problematic here since it causes the statements on the
right hand side to be dereferenced immediately even if the repo or .git
target was not specified.  When the source is checked out from a git clone,
the .gitrev file does not exist, but the '$(shell cat .gitrev)' sequence is
executed every time make is called and it produces an error message.  Also,
the '$(shell mktemp -d tmprepo.XXXXXX)' sequence is  executed every time
make is called, which creates a new unique tmprepo.XXXXXX directory each
time.

So, let's rework this target so that the := notation is avoided, and also
to make it a little more efficient by avoiding an unnecessary checkout of
the repository.

There are two functional changes: 1) the temporary directory will just be
called ".repo_tmp" and it will be overwritten each time 'make repo' is
called, and 2) the checked out branch will keep its default name rather
than adopting a new name 'working'.  The user can create a new branch if
they desire and the original remote branch state is already available in
origin/master.
Makefile: checkout all missing files for repo target

Rather than try to maintain a list of the file differences between the
git repository and the source distribution tarball, just get a listing from
git itself and use it to check out the missing files.
Merge pull request #58 from drafnel/master

Makefile: rework repo creation target