It can be useful for the caller to know which update commands will be
sent to the server before the packfile is pushed up. git does this via
the pre-push hook.

We don't have hooks, but as it adds introspection into what is
happening, we can add a callback which performs the same function.

Implement git_submodule_set_branch.

Handle hide-then-push in the revwalk

As it seems it's not quite clear what it means to push a commit, try to
be more explicit about adding a new root and that we may not see this
commit if it is hidden.

Make sure to pack referenced objects for non-branches

Add a custom param to git_smart_subtransport_definition

note: use a git_buf to return the default namespace

Allow merges of files (and trees) with whitespace problems/fixes