largefiles: allow to run 'debugupgraderepo' on repo with largefiles The extensions wrap the necessary function to ensure the 'largefiles' requirements won't be dropped. It is now possible to run `hg debugupgraderepo` on a repository with largefiles.

upgraderepo: allow extension to register preserved requirements Some requirement does not directly result from config and needs more advanced logic to be preserved. The current example is 'largefiles'. We add a hook point in the upgrade code so that extensions can handle these cases. The 'largefiles' extension will use it in the next changeset.

revset: make follow() accept empty startrev This is the same behavior as ancestors(emptyset).

revset: alias follow(startrev=rev) to ancestors(rev) This seems natural given 'log -frREV' (with no file pattern) is equivalent to 'log -frREV *'.

revset: make follow() accept keyword arguments Also renamed the argument from 'pattern' to 'file' conforming to followlines().

revset: make follow() accept multiple startrevs The diff might look slightly complicated, but the initial "c = repo['.']" was effective if rev = None.

dagop: use heap to compute max rev in filectxancestors()