Allow reducing size of the left column

**Is your feature request related to a problem? Please describe.**
Vertical split at the "Status [1]" section is at 50% which is suboptimal in many cases. List of files is likely to take (much) less space compared to the diff.

**Describe the solution you'd like**
Either resize it dynamically or make it user-configurable. Or, allow changing on the fly.

**Describe alternatives you've considered**
Reposition screen from vertical to horizontal orientation or purchase a wider monitor.

**Additional context**