help: expand the extensions topic

help: refactor extensions listing, and show enabled ones in the dedicated topic

help: adding a new help topic about extensions Currently listing non-enabled extensions and a short introductory text. Thanks to Dan Villiom Podlaski Christiansen for the preliminary proof-of-concept code for listing available extensions.

bookmarks: add appropriate locking (issue1691) - make updates of bookmark state locked and atomic - wrap commit so that commit and bookmarks happen under the same lock

filemerge: fix internal:dump

hgweb: deny cloning a subpath of a repo

hgweb: web.encoding should override encoding.encoding (issue1183)

bdiff: fix compile with GCC -ansi (issue1690)

tags: silence warning about unknown tags This is mostly a nuisance and can happen legitimately with pull -r, etc.

rollback: minor clarification (issue828)