serve: unify cmdutil.service() calls of commandserver and hgweb

hgweb: extract factory function of httpservice object The next patch will merge the cmdutil.service() calls of both commandserver and hgweb. Before doing it, this patch wipes out the code specific to hgweb from commands.serve().

hgweb: move httpservice object from commands module This avoids the deep import of hgweb.server at the commands module.

merge: move almost all change/delete conflicts to resolve phase (BC) (API) We have finally laid all the groundwork to make this happen. The only change/delete conflicts that haven't been moved are .hgsubstate conflicts. Those are trickier to deal with and well outside the scope of this series. We add comprehensive testing not just for the initial selections but also for re-resolves and all possible dirstate transitions caused by merge tools. That testing managed to shake out several bugs in the way we were handling dirstate transitions. The other test changes are because we now treat change/delete conflicts as proper merges, and increment the 'merged' counter rather than the 'updated' counter. I believe this is the right approach here. For third-party extensions, if they're interacting with filemerge code they might have to deal with an absentfilectx rather than a regular filectx. Still to come: - add a 'leave unresolved' option to merges - change the default for non-interactive change/delete conflicts to be 'leave unresolved' - add debug output to go alongside debug outputs for binary and symlink file merges

test-merge-changedelete.t: print out debugmergestate We're going to use this to verify the merge state in upcoming patches.

debugmergestate: also recognize change/delete conflicts in the merge state We're going to use this for tests in upcoming patches.

debugmergestate: print out null nodes as 'null' This is so much easier to read than a long string of zeroes, and we're going to have a lot more of these nodes once change/delete conflicts are part of the merge state.

test-merge-force.t: check .orig files separately We're going to soon compare the output of all the non-orig files before and after a resolve, and this makes that more convenient. The .orig files are obviously going to differ between the two.