Give more space to right-side diff view

[hongquan]

In the "inspect commit" view, the left pane (for commit message and list of changed files) and the right pane (for viewing diff) each takes a half of the window. It is not reasonable because the right pane has more content to show

This PR is to give more space to the right pane.

It changes the following:

• Change the layout constraints to reduce the width of the left pane.
• Provide different constraints for different window size, so that the left pane width is reasonable.

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