The beginnings of patch application from an existing (diff-created)
git_patch object: applies the hunks of a git_patch to a buffer.

The `giterr_set_str` does not actually honor `GITERR_OS`.  Remove
the documentation that claims that we do.

Provide a new merge option, GIT_MERGE_TREE_FAIL_ON_CONFLICT, which
will stop on the first conflict and fail the merge operation with
GIT_EMERGECONFLICT.

This is to be returned when the operation which the user asked for is
not possible to do on a directory.

We've been using EINVALIDSPEC for a while to mean this, but that name
is too specific. Introduce this to be more explicit.

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.

This can be used by tools to show mesages about failing to communicate
with the server. The error message in this case will often contain the
server's error message, as far as it managed to send anything.

Walk up the tree to mkdir, which is less immediately efficient,
but allows us to look at intermediate directories that may need
attention.