* lisp/emacs-lisp/track-changes.el (track-changes-inconsistent-state-p):
New function.
* lisp/progmodes/eglot.el (eglot--track-changes-signal): Use it.
Arrange for the library to be usable on older Emacsen, which
includes reducing the noise when `before/after-change-functions`
are badly paired or missing.
Also, since the signal function receives the distance (for `:disjoint`),
we don't need `track-changes--disjoint-threshold`: the signal
function can simply do nothing when the distance is smaller than
the threshold it wants to use.
* lisp/emacs-lisp/track-changes.el: Prepare header for ELPA.
(track-changes--tracker, track-changes--state): Don't use `:noinline`,
so as to be compatible with Emacs<27.
(track-changes-record-errors): New variable.
(track-changes--recover-from-error): Use it. Record only the last 20
keys and the last 50 stack frames in the error log.
(track-changes--disjoint-threshold): Delete variable.
(track-changes--before): Don't use it any more.
* lisp/progmodes/eglot.el (eglot--track-changes-signal):
Coalesce disjoint changes nearer than what used to be coalesced because of
`track-changes--disjoint-threshold`.
* lisp/emacs-lisp/track-changes.el (track-changes--before):
Widen the buffer before accessing it with positions
potentially outside the beg..end region.
This new package provides an API that is easier to use right than
our `*-change-functions` hooks.
The patch includes changes to `diff-mode.el` and `eglot.el` to
make use of this new package.
* lisp/emacs-lisp/track-changes.el: New file.
* test/lisp/emacs-lisp/track-changes-tests.el: New file.
* doc/lispref/text.texi (Tracking changes): New subsection.
* lisp/progmodes/eglot.el: Require `track-changes`.
(eglot--virtual-pos-to-lsp-position): New function.
(eglot--track-changes): New var.
(eglot--managed-mode): Use `track-changes-register` i.s.o
`after/before-change-functions` when available.
(eglot--track-changes-signal): New function, partly extracted from
`eglot--after-change`.
(eglot--after-change): Use it.
(eglot--track-changes-fetch): New function.
(eglot--signal-textDocument/didChange): Use it.
* lisp/vc/diff-mode.el: Require `track-changes`.
Also require `easy-mmode` before the `eval-when-compile`s.
(diff-unhandled-changes): Delete variable.
(diff-after-change-function): Delete function.
(diff--track-changes-function): Rename from `diff-post-command-hook`
and adjust to new calling convention.
(diff--track-changes): New variable.
(diff--track-changes-signal): New function.
(diff-mode, diff-minor-mode): Use it with `track-changes-register`.
