[Expo-tech] git conversion

[Mark Shinwell]

I've spent rather too long on this today and yesterday, so will continue
next week.  I've constructed a git repo which is mostly done, with tags
placed after each year; it starts in 2001 from the CVS commits.  However I
think I've made a mistake during the rebasing with some of the top-level
files which I need to fix before the whole dataset will process.  I'm also
going to try processing it at each of the tags (post-2001, post-2002 etc).

The import of the hg changesets was more problematic than expected and a
lot of conflicts had to be fixed.

I have managed to move the ARGE 2012 updates, which previously weren't
merged until after Expo 2013 (confusingly), so they are actually before
Expo 2013.  There may still be some places where changesets aren't in the
correct by-year-chronological place but I don't think this should be on a
large scale, and it isn't likely to be worth fixing I suspect (the other
cases I've seen would involve splitting changesets, which is likely to have
other ramifications).  Going forward I suggest we try to keep data in the
right place between year boundaries, but not worry about the exact ordering
within a year.

Keeping this as a linear history branch is probably the right thing to do,
but I'm not completely certain yet.  I also think we should carefully
consider the proposed workflow and how to keep the repository in a good as
state as possible automatically.  This should also include discussing with
other people (maybe just ARGE?) to ensure that they're on the same page.
Large imports in blocks from other sources aren't good for keeping things
in order, as seen in the 2012 ARGE example.

The combined git repo and working copy is about 80% of the size of the
Mercurial one.  I decided against filtering every changeset as I'll have to
recreate all the tags again most likely, and it turns out the various
useless files were mostly removed long ago (and I also prevented some of
them getting in right at the start of the history).

Mark
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