bug#37681: 27.0.50; rainbow-mode highlights some URL fragments when it should not

[Nate Eagleson]

If a URL contains a fragment identifier that can be interpreted as a
hexademical color, rainbow-mode will highlight it as such.

For instance, in the URL 'http://localhost/test#debug', the string
'#deb' is highlighted as a color.

Recipe:

Run `emacs -Q` at a bash prompt.

Do `M-x load-file` and load rainbow-mode.el 1.0.1 (in my setup it's at
~/.emacs.d/elpa/rainbow-mode-1.0.1/rainbow-mode.el).

In the *scratch* buffer, do `M-x rainbow-mode`.

Insert "http://localhost/test/#debug";.

Note that the substring "#deb" is highlighted.


In GNU Emacs 27.0.50 (build 4, x86_64-apple-darwin18.7.0, NS appkit-1671.60 
Version 10.14.6 (Build 18G87))
 of 2019-08-21 built on NXT-NEAGLESON
Repository revision: 50dc4ca8d02a466a7236765edf83ae7cfb02d74c
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.6

Recent messages:
Saving file /Users/neagleson/daily-log/2019-10-09.txt...
Wrote /Users/neagleson/daily-log/2019-10-09.txt
Total time worked this week: 23 hours, 54 minutes
Mark set
Saving file /Users/neagleson/daily-log/2019-10-09.txt...
Wrote /Users/neagleson/daily-log/2019-10-09.txt
Saving file /Users/neagleson/daily-log/2019-10-09.txt...
Wrote /Users/neagleson/daily-log/2019-10-09.txt
Reverting buffer ‘config.xml’. [7 times]
ESC ESC M-b is undefined

Configured using:
 'configure --with-ns --with-imagemagick --with-modules
 --enable-silent-rules PKG_CONFIG_PATH=/opt/local/lib/pkgconfig
 LDFLAGS=-L/usr/local/lib LDFLAGS=-L/usr/lib
 CPPFLAGS=-I/usr/local/include
 CPPFLAGS=-I/usr/local/Cellar/libxml2/2.9.7/include/libxml2 CC=clang
 OBJC=clang 'CFLAGS=-g -O2''

Configured features:
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MODULES THREADS PDUMPER LCMS2 GMP

Important settings:
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Major mode: Markdown

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~/.emacs.d/site-lisp/magit-svn hides 
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~/.emacs.d/site-lisp/css-mode hides 
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/Users/neagleson/.emacs.d/elpa/faceup-20170925.1946/faceup hides 
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