Add a command-line option to override the default log file path.

gitui --logging stays unchanged;
gitui --logfile path/to/file.log logs to path/to/file.log;
gitui --logging --logfile path/to/file.log is equivalent to using just --logfile.

This Pull Request fixes/closes #2528.

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