util: teach stringmatcher to handle forced case insensitive matches The 'author' and 'desc' revsets are documented to be case insensitive. Unfortunately, this was implemented in 'author' by forcing the input to lowercase, including for regex like '\B'. (This actually inverts the meaning of the sequence.) For backward compatibility, we will keep that a case insensitive regex, but by using matcher options instead of brute force. This doesn't preclude future hypothetical 'icase-literal:' style prefixes that can be provided by the user. Such user specified cases can probably be handled up front by stripping 'icase-', setting the variable, and letting it drop through the existing code.

revset: point to 'grep' in the 'keyword' help for regex searches The help for 'grep' already points to 'keyword'.

help: explain that revsets can be used where 1 or 2 revs are wanted We did not seem to document that one can do things like "hg up :@" where the last revision of the revset ":@".

help: explain what the term "revset" means We refer to revsets in a few places (e.g. in "hg help config"), but we never explained what they are. Until now.

help: merge revsets.txt into revisions.txt Selecting single and multiple revisions is closely related, so let's put it in one place, so users can easily find it. We actually did not even point to "hg help revsets" from "hg help revisions", but now that they're on a single page, that won't be necessary.

tests: use `hg help dates` instead of `hg help revs` in test The revisions help is already long and will get longer, so switch to another short and stable topic.