simplify magit-parse-log-line

* A small change in `magit-log-oneline-re' allows removing an ugly
  kludge here.
* make internal function `match-style-string' all cases
* use cl-mapcan
* rearrange and align a bit

This is in preparation of merging #619 which featured similar changes
(though not exactly the same) but mixed them in the same commit with the
addition of a new feature.
colorize label in `magit-reflog'