Support colorising line line-number-mode rather than writing to the fringe/gutter

[sten0]

@dgutov Continuing from #103

Regarding the coloring of line numbers, I'm not really sure it's possible
(the new feature is implemented on the C level, and it's not very
customizable). You're welcome to file a separate feature request in Emacs
or here (I will pass it on then).

I suspect one of the following two (naïve) approaches would work, since
line-number-mode respects Custom Themes.

1. If lnm inherits face attributes from the theme, then a theme could
   provide a lnm-specific variable (defaults to inheriting foreground &
   background face from theme). That variable would be dynamically
   modified by diff-hl to indicate state. Lnm would need to be modified
   to use this new theme variable instead of the global foreground &
   background face.

2. Basically the inverse of the above. Lnm operates at a lower level
   and dumps data which is then rendered by the interface (obviously I
   don't know Emacs internal architecture well at all). In this case Lnm
   would need to be modified to signal to the interface that it's data is
   different (maybe it already does?)...so then it can be rendered using a
   different path. Diff-hl would intercept this data post-custom theming
   but before the interface rendered it, and would overwrite the face
   attributes applied by the custom theme.

[dgutov]

line-number-mode? Is that the one that writes to current line number at the beginning of the mode-line?

[dgutov]

If you mean display-line-numbers-mode (and I agree that conflict of names in unfortunate), then I don't think either of the approaches will work currently. We can file a feature request, though.

[dgutov]

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36472