Historic git tags added

[Olly Betts]

We only had git tags for releases 1.2.20 and later which is a bit
unhelpful when digging back in the history and I spotted recently
this has lead me on a few past occasions to incorrectly conclude
something was introduced by 1.2.20 when it was actually rather earlier.

To address this I've added git tags for 1.2.0 to 1.2.19 inclusive and
also 1.0.23.

I didn't see a way to easily automate this, so I just found what looked
like the final commit included in each release with a quick check
against the tarball when it seemed unclear so they should be accurate,
but it's not guaranteed like it is for newer tags where the tagging
happens as part of the release process.

I'll probably add tags for earlier releases and 1.1.x development
snapshots at some point too.

Cheers,
    Olly