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.