Revcheck changes

Hi,

Today I merged the first batch of revcheck changes. The first batch
aims to remove revcheck code duplication from various repositories, in
an almost compatible way. There may be some minor differences, as file
counting and revtag parsing got more strict.

Following this, is planned other changes that intentionally changes
output, adding and removing files from tracking, changing the
implementation of [skip-revcheck] and (hopefully) fixing all TODOs
noted in code deduplication effort.

The development occurred at https://github.com/php/doc-base/pull/174 ,
and further changes will be discussed in other, separated PRs, in this
repository. I suggest Watching this repository to be notified and
discuss these other changes.

André L F S Bacci