If the user is running `git commit` to resolve a conflict, it's
dangerous to just save the metadata from the filesystem. It's possible
that they just fixed a conflict in the metadata file, but didn't update
the on-disk metadata to match.

(I know it's possible, because I did this!)

Our normal mode of operation, of just saving the metadata during a
commit to match whatever's on-disk, would wipe out their careful
conflict resolution! (On the other hand, just overwriting the on-disk
metadata might not be the best idea either.)

But the point of a conflict is that the changes between two different
branches *can't* be resolved automatically, and requires manual
intervention. Given that situation, asking them to resolve the
difference manually seems to be the most obvious option.

Previously after first metastore commit, metastore -c would show

    ./.metadata:	added

which was misleading.

Now metastore -s is run twice if .metadata is not in repository yet.

Fixes #26.