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:

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