Add --logfile

[acuteenvy]

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

[cruessler]

@extrawurst I can have a look at this PR this week!

[cruessler]

@extrawurst I reviewed the PR on my Ubuntu machine and noticed two things:

• I was wondering whether --logfile needed the short version -f as the latter conventionally also is short for --force.
• The PR contains spaces instead of tabs, as far as I can see in my editor, but cargo fmt doesn’t seem to complain, neither on my machine nor on CI.

Otherwise, I’d say this is good to go!