allow update to switch to a different branch if the repo is clean

Explicitly use REVLOGV0 instead of 0 in bundlerepo.

Define and use nullrev (revision of nullid) instead of -1.

Correct optimization from 3464f5e77f34; add a test. If there are two (or more) heads that point to the same .hgtags node, we can safely skip parsing the file in all but the last head. (In 3464f5e77f34, we were parsing the file in the first head and skipping all the others.)

log speedup: don't call revlog.node if we don't have to

log speedup: use list.extend to avoid an explicit loop

log speedup: use parentrevs in log and changeset_printer.show

log speedup: set quiet/verbose/debugflag in the fake ui object

log speedup: use only ui.write in changeset_printer.show - ui.status is not very useful, since this code is never executed when ui.quiet is true. - explicitly checking for ui.debug allows a regular hg log to run without looking at the manifest index - the calls to ui.note were converted for consistency

log speedup: walkchangerevs: filter the files only if we need them This speeds up hg log and significantly reduces memory usage (max RSS goes from ~92MB to ~21MB on the kernel repo), since we no longer store all the revisions in the cache.