Prevent cache of diff-mode buffers to grow without bounds

Prevent cache of diff-mode buffers to grow without bounds

Previously, these " *diff-syntax:..." buffers were never removed.
Now we discard the least recently used half of them every hour.

* lisp/vc/diff-mode.el (diff--cached-revision-buffers)
(diff--cache-clean-interval, diff--cache-clean-timer, diff--cache-clean)
(diff--cache-schedule-clean, diff--get-revision-properties): New.
(diff-syntax-fontify-hunk): Use diff--get-revision-properties.

(cherry picked from commit 4d57187a248d3243dcc8b5da5d8365cb1b54a347)