gitk crashes if the diff contains certain unicode characters

Looks like certain characters cause gitk to crash when it tries to render a diff.

How to repro:

	git init
	echo foo >foo
	git add foo
	git commit -m foo
	# The following assumes bash builtin echo
	echo -e '\u2754' >foo
	gitk

Now `foo` should contain the character U+2754 encoded as utf-8.

If on gitk you click on "Local uncommitted changes", it'll crash with the following message:

	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:  4220
	  Current serial number in output stream:  4247

Note that if I now commit, gitk will crash at startup because the diff with the character are on the last commit (which gitk selects by default)

	git commit -am crash
	gitk # crashes
	gitk HEAD^ # works fine

After that, you can add a couple more commits and gitk works fine, unless you click on one of the commits whose diff contain the character. eg:

	echo foo >foo
	git commit -am foo
	echo bar >foo
	git commit -am bar
	gitk

Version / System:

	$ git --version
	git version 2.20.1
	$ lsb_release -d
	Description:    Ubuntu 19.10