[RFC] custom strategies in builtin-merge

Hi,

We talked with Dscho about how would it be possible to allow custom
strategies in builtin-merge and I have a question here.

The problem is that currently merge strategies are always named
git-merge-foo, but not all git-merge-foo is a merge strategy.

So we talked about two solutions here:

1) Maintain a list of commands that has a git-merge- prefix, but not a
strategy. This list would currently contain "base, file, index,
one-file and tree".

2) Require custom strategies to have a different naming scheme, like
if "foo" is a custom strategy, then it would have to be named
git-merge-custom-foo, _not_ git-merge-foo.

Both are doable (I prefer 1) a bit), but I thought it's better to ask
first before I implement any of them.

So, comments?

Thanks.