* push: respect `branch.*.merge` when push default is upstream

This is a function that's solely used in testing and has no
dependencies into git2_hooks and panics on common errors.
It's a much better fit for git2-testing.

git supports relative values in core.hooksPath.

`man git-config`:

> A relative path is taken as relative to the directory where the hooks are
> run (see the "DESCRIPTION" section of githooks[5]).

`man githooks`:

> Before Git invokes a hook, it changes its working directory to either
> $GIT_DIR in a bare repository or the root of the working tree in a >
> non-bare repository.

I.e. relative paths in core.hooksPath in non-bare repositories are always
relative to GIT_WORK_TREE.

There is a further exception; I believe this is not considered for path
resolution:

> An exception are hooks triggered during a push (pre-receive, update,
> post-receive, post-update, push-to-checkout) which are always executed
> in $GIT_DIR.