feat: use git commit --verbose template in external editor

[lljbash]

This Pull Request closes #2619.

It changes the following:

• When opening the external editor on a commit message, use template from git commit --verbose which includes full diff of the commit.

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog