bug#32730: 26.1.50; gnus should font-lock patches coming from git send-email

[Tom Tromey]

Most gdb patches are sent using git send-email.  This appear in gnus as
plain text.  However, it seems to me that it's relatively easy to
recognize patches sent from git, and that gnus could font-lock the body
of the patch.

Tom


In GNU Emacs 26.1.50 (build 11, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-09-06 built on pokyo
Repository revision: 6c616e465cc83cf376d4df75f9c4afdbdf0ef6ca
Windowing system distributor 'Fedora Project', version 11.0.11906000
System Description:     Fedora release 28 (Twenty Eight)

Recent messages:
Quit
mwheel-scroll: End of buffer [3 times]
Mark set
C-h C-g is undefined
Type C-x 1 to delete the help window, C-M-v to scroll help.
Mark saved where search started

Mark saved where search started [4 times]
scroll-down-command: Beginning of buffer
scroll-up-command: End of buffer [2 times]

Configured using:
 'configure --prefix=/home/tromey/Emacs/install --with-modules'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GSETTINGS GLIB NOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
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  value of $XMODIFIERS: @im=ibus
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Major mode: Tcl

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Load-path shadows:
/home/tromey/.emacs.d/elpa/bubbles-0.5/bubbles hides 
/home/tromey/Emacs/install/share/emacs/26.1.50/lisp/play/bubbles

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Memory information:
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