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()

dagop: add smartset interface to filectxancestors() The original filectx API is kept public since we'll need it to walk ancestor (rev, match) pairs efficiently. The current implementation scans ancestors twice for 'hg log -fp FILE'.

overlayworkingctx: add `_checkexist(path)` This is in preparation to switch this class to inheriting (and being based off a) `commitctx` instead of a `workingctx`. `filectx` has no `exists` function, so this is how we'll fall back in that case. Differential Revision: https://phab.mercurial-scm.org/D1237

overlayworkingctx: fix a bad reference to `self._path` Differential Revision: https://phab.mercurial-scm.org/D1236