Merge from origin/emacs-27

Merge from origin/emacs-27

dd5756436c Move more logic to vc-ignore from vc-default-ignore
2aed279be1 Warn about the likes of "[:alnum:]" in regexps
0273f261a7 Don't write absolute filenames and duplicate strings to CV...
d7c22338d2 Fix cursor-sensor--detect when current buf != selected win...
2e39fc83bb * doc/emacs/sending.texi (Mail Sending): Fix index entries.
b410f902d5 Document 'message-send-mail-function' in the Emacs manual
ac0546612d Fix reference to 'message-send-and-exit' in Emacs manual
cd6a9b8f65 Skip shell prompt on current line in Eshell even if it's p...