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