merge: use skip_prefix to parse config key

Instead of using `starts_with()`, the magic number 7, `strlen()` and a fair number of additions to verify the three parts of the config key "branch.<branch>.mergeoptions", use `skip_prefix()` to jump through them more explicitly. We need to introduce a new variable for this (we certainly can't modify `k` just because we see "branch."!). With `skip_prefix()` we often use quite bland names like `p` or `str`. Let's do the same. If and when this function needs to do more prefix-skipping, we'll have a generic variable ready for this. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>