bug#1254: 23.0.60; linum-mode display problem with variable-pitch font

[Chong Yidong]

> 1. emacs -Q
> 2. C-SPC M-< M-w C-y C-y C-y (copy the *scratch* text so the buffer has
> more than ten lines)
> 3. M-x variable-pitch-mode
> 4. M-x linum-mode
>
> The rightmost digits in the double digit line numbers are partly cut off

Currently, Emacs assumes that the pixel width of the left margin is
simply the width of the left margin, in character glyphs, times the
default width of a glyph in pixels.  This assumption obviously runs into
problems if you display a variable-width font in the left margin.

Fixing this in redisplay is not easy, and at this stage of the release
process I'd prefer not to touch that part of the code.  (For instance,
we can't just let the left margin have an arbitrary pixel width, because
window resizing assumes otherwise.)

I suggest a workaround: make the `linum' face inherit the face
"fixed-pitch".  Markus, WDYT?