More customisable log output

[Philip Jackson]

Hi,

I've added support for customisable log output which (by default)
looks like this:

http://www.shellarchive.co.uk/images/magit-log-porn.png

Personally I don't use the graph output much (probably never work on a
complex enough tree) so I dimmed it by default, perhaps the wrong
thing to do.

It's customisable via `magit-present-log-line-function',
`magit-log-graph', `magit-log-message' and `magit-log-sha1'. The patch
fixes an issue with git-svn refs not being highlighted but is slightly
more delicate at the moment (parens at the start of a commit message
will cause issues).

Cheers,
Phil