Reconcile erc-stamp--date-stamps when merging buffers

Reconcile erc-stamp--date-stamps when merging buffers

* etc/ERC-NEWS: Mention new face `erc-information'.
* lisp/erc/erc-button.el (erc-button-add-buttons): Skip buttonization
when the "msg prop" `erc--skip' is present and contains the symbol
`button'.  Set `alist' to nil in the same guard condition as a
roundabout way of suppressing further processing.
* lisp/erc/erc-networks.el (erc--insert-admin-message): Forward
declaration.
(erc-networks--insert-transplanted-content)
(erc-networks--transplant-buffer-content): Replace former with latter.
Change signature to take source and destination buffers as parameters.
(erc-networks--transplant-target-buffer-function): New function-valued
variable.
(erc-networks--target-transplant-in-progress-p): New variable, a flag
for downstream code to detect when a transplant is underway.
(erc-networks--reclaim-orphaned-target-buffers): Defer to
`erc-networks--transplant-target-buffer-function' to handle the actual
transplant business.  Crucially, kill the buffer afterwards instead of
beforehand.  If new buffer-association bugs emerge related to the
combining of old or renamed target buffers, this reordering may be at
fault.
(erc-networks--copy-over-server-buffer-contents): Pass old and new
buffers to `erc-networks--insert-transplanted-content'.
* lisp/erc/erc-stamp.el
(erc-stamp--defer-date-insertion-on-post-modify): Set `fn' slot of
`erc-stamp--date' instance to `ignore' when running the actual
callback in order to conserve a little space.
(erc-stamp--date-mode): Add and remove hook members for
`erc-networks--copy-server-buffer-functions' and
`erc-networks--transplant-target-buffer-function'.
(erc-insert-timestamp-left-and-right): Always clear
`erc-timestamp-last-inserted-right' to ensure a right stamp
accompanies every date stamp.
(erc-stamp--dedupe-date-stamps)
(erc-stamp--dedupe-date-stamps-from-buffer)
(erc-stamp--dedupe-date-stamps-from-target-buffer): New functions.
Date stamp behavior was revamped as part of bug#60936.
* lisp/erc/erc.el (erc-informational): New face.
(erc--insert-admin-message): New function to hide some "msg prop"
complexity from "upstream" libraries, like `erc-networks', and thus
avoid more forward-declarations.  A less smelly approach would be to
devise a general interface owned by such libraries, or erc-common,
that `erc-mode' could then hook into on init.
(erc-display-message-highlight): Make face matching more limber to
accommodate the convention of face names lacking a "-face" suffix.
(erc-message-english-graft): New variable.
(erc-kill-channel): Inhibit execution of hook when
`erc-networks--target-transplant-in-progress-p' is non-nil.
* test/lisp/erc/erc-networks-tests.el
(erc-networks--rename-server-buffer--no-existing--orphan)
(erc-networks--rename-server-buffer--existing--reuse)
(erc-networks--rename-server-buffer--local-match)
(erc-networks--rename-server-buffer--local-nomatch): Use helper to
initialize markers.
* test/lisp/erc/erc-stamp-tests.el (erc-stamp--dedupe-date-stamps):
New test.  (Bug#70928)

(cherry picked from commit fee637468b5e72fd6fcd2c96c42622455db5fb16)