hgweb: ensure _navseq yield strictly increasing numbers This is not hard and allows to drop hack in the customer code.

hgweb: better names for _navseq arguments The old names were misleading.

hgweb: drop recursivity in _navseq This is totally not needed.

hgweb: move the `seq` function out of the revnavgen scope There is not reason for it to be a in there. And this function could use a major reworks.

hgweb: simplify return value creation in for navgen We now have access to better syntax allowing a clearer version.

hgweb: do not access first changeset with a string There is not reason not to use an int

refactoring: use unlinkpath with ignoremissing

docs: "deprecate" checkchanged and checkconflicts in merge-tool configuration These settings were replaced by check=changed and check=conflicts in a912f26777d3. There is no reason to announce two different ways to achieve the same. The old way should be kept but not announced.