Set alternate faces for some line numbers (bug#27734)

* lisp/faces.el (line-number-major-tick, line-number-minor-tick):
New faces.
* lisp/cus-start.el (display-line-numbers-major-tick)
(display-line-numbers-minor-tick): Add customization info.
* lisp/frame.el: Add `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick' to list of variables which
should trigger redisplay of the current buffer.

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-major-tick>
<display-line-numbers-major-tick>: Defvar new options.
(syms_of_xdisp) <line-number-major-tick, line-number-minor-tick>:
Defsym new faces.
(maybe_produce_line_number): Use new faces for line numbers
that are multiple of `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick'.

* etc/NEWS (value): Announce new feature.
* doc/emacs/display.texi (Display Custom): Describe it.  Wording by
Robert Pluim <rpluim@gmail.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index beb15776ef6..4a323002686 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -559,6 +559,13 @@ network connection information (in addition to the host name).
 now prompts the user for the directory containing the desktop file.
 
 +++
+** display-line-numbers-mode
+*** New faces 'line-number-major-tick' and 'line-number-minor-tick',
+and customizable variables 'display-line-numbers-major-tick' and
+'display-line-numbers-minor-tick' can be used to highlight the line
+numbers of lines multiple of certain numbers.
+
++++
 ** winner
 *** A new variable, 'winner-boring-buffers-regexp', has been added.