Rename GIT_EMERGECONFLICT to GIT_ECONFLICT

Rename GIT_EMERGECONFLICT to GIT_ECONFLICT

We do not error on "merge conflicts"; on the contrary, merge conflicts
are a normal part of merging.  We only error on "checkout conflicts",
where a change exists in the index or the working directory that would
otherwise be overwritten by performing the checkout.

This *may* happen during merge (after the production of the new index
that we're going to checkout) but it could happen during any checkout.