F3 should open the external diff viewer instead of showing "No string specified to look for!"

[mstv]

Do you want to request a feature or report a bug?
feature

What is the current behavior?
Shortcut "F3" opens the external diff viewer only if a file list has the focus.
If it's hit with the internal diff focused and no search was started before, the error message "No string specified to look for!" will pop up.

What is the expected behavior?
Shortcut "F3" opens the external diff viewer, too, while the internal diff or the commit dialog has the focus unless a text search was started in the internal diff before.

I'm going to provide my first pull request.

Environment you encounter the issue:

• GitExtensions version: 2.51
• GIT version: N/A
• OS version: Win7

Did this work in previous version of GitExtensions (which)?
no (new feature)