Save unified diff can cause TortoiseGit to not respond

What steps will reproduce the problem?

1. Checkout a large project, I for example work on php-src (http://git.php.net/?p=php-src.git;a=summary) so diffing can take some time
2. Context menu > TortoiseGit > Diff
3. While it loads, click Save Unified Diff
4. TortoiseGit stops responding

What is the expected output? What do you see instead?

I don't expect TortoiseGit to freeze, but instead leave buttons like this disabled until the files in the diff window are loaded before allowing the button to be clickable

What version of TortoiseGit and Git are you using? On what operating system?

TortoiseGit 2.3.0.0 Git 1.9.5.msysgit.1 Windows 7 x64