I want an efficient way to get information about the commits of a repo.
So far, our Commit model contains a mix of immutable and mutable fields, and this means we need to throw out
commits whenever we refresh, because one of the mutable fields may have changed.
The commit store will store a new model, ImmutableCommit which never changes so we can just continue adding
commits to the store without worrying about invalidating any of it.
One use case for this store is the ability to determine if one commit is an ancestor of another, which will
help us colour the commits against each of our branches in the local branches view. Without an in-memory
store, we would need to make one git call per commit which would be super slow.
If this store proves useful, we could switch to using it as the source of truth for our commits, with
mutable stuff handled separately.
The main reason for doing this (besides the reasons given for Status in the
previous commit) is that it allows us to easily convert from TodoCommand to
Action and back. This will be needed later in the branch. Fortunately,
TodoCommand is one-based, so this allows us to add an ActionNone constant with
the value 0.
This is unrelated to the changes in this PR, but since we are doing the same
thing for the commit.Action field in the next commit, it makes sense to do it
for Status too for consistency. Modelling this as an enum feels more natural
than modelling it as a string, since there's a finite set of possible values.
And it saves a little bit of memory (not very much, since none of the strings
were heap-allocated, but still).
