merge: use skip_prefix to parse config key

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>