vc-hg: prompt for branch to merge

* lisp/vc/vc-hg.el (vc-hg-merge-branch): Prompt for revision to merge.
(vc-hg-revision-table): Use branches, tags and bookmarks as competion
candidates.

* etc/NEWS: Mention changes of vc-hg.el

* doc/emacs/maintaining.texi (Switching Branches): Mention 'hg update'
command.
(Merging): Mention 'hg merge' command.

This fixes bug#22860
1 job for master in 90 minutes and 2 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #4394
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/isearch-tests.log
ELC lisp/jit-lock-tests.elc
GEN lisp/jit-lock-tests.log
ELC lisp/json-tests.elc
GEN lisp/json-tests.log
ELC lisp/jsonrpc-tests.elc
GEN lisp/jsonrpc-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds