bug#56682: Fix the long lines font locking related slowdowns

[Stefan Monnier]

Before using a blunt tool like the forced-narrowing now in
`feature/long-lines-and-font-locking`, I think we should try and figure
out *why* the recipe below is so slow.

For example, the behavior surprises me because I can't see why `M-x`
should cause any font-locking at all in the `long-line.xml` file.

[ Other side note: IIRC `nxml-mode` uses font-lock and syntax-propertize
  in somewhat unusual ways (it started doing all of that work "by hand"
  in its own way, and only later was it coerced to try and play along
  with that "standard" infrastructure), so the problem may be specific to
  nxml-mode.  ]


        Stefan