coloured emojis and regional indicators kill gitk

[Bruno Haible]

Hi,

gitk crashes when displaying a specific commit in the gnulib git repo.

How to reproduce:
$ gitk
Navigate to the commit 2995fb5e993a5d7434d96465758087b35a1488ac
(title "unilbrk: Update handling of zero-width joiner for Unicode 10.0.0.")
=>
X Error of failed request:  BadLength (poly request too large or internal Xlib 
length error)
  Major opcode of failed request:  139 (RENDER)
  Minor opcode of failed request:  20 (RenderAddGlyphs)
  Serial number of failed request:  12728
  Current serial number in output stream:  12796

This is a known bug in the Xft library [1][2].

I noticed that it also happens for "regional indicator" characters.
(Pairs of regional indicators render as a region's flag.)

Sorry, I did not know about this bug before checking in the commit.

Bruno

[1] https://unix.stackexchange.com/questions/629281/
[2] https://bugs.launchpad.net/ubuntu/+source/git/+bug/1852985