bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Juri Linkov]

X-Debbugs-CC: Stefan Monnier <monnier@iro.umontreal.ca>

The simplest test case to reproduce this is to eval in emacs -Q:

(with-temp-buffer
  (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
  (c-mode)
  (font-lock-ensure 21 (point-max))
  (get-text-property (point-min) 'face))

that returns
=> font-lock-preprocessor-face

This means that `font-lock-ensure' fontifies not only the given region,
but also the whole buffer.

OTOH,

(with-temp-buffer
  (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
  (c-mode)
  (font-lock-fontify-region 21 (point-max))
  (get-text-property (point-min) 'face))

returns `nil' that means that it fontifies only within the specified boundary.

This difference is essential for bug#33567: Syntactic fontification of diff 
hunks
where fontifying the whole buffer might slow down fontification of diff hunks
that are small parts of the whole buffer.