Create log files in case of errors

[saschanaz]

Errors may not be visible when git commands run behind the scene by tools e.g. VSCode or GitHub Desktop. Such errors may still be useful for debugging, and in that case a file logging option can help.

[glandium]

My proposal for this would be to allow to add ">file" in cinnabar.log or GIT_CINNABAR_LOG. So that you could set:

• >foo to output everything cinnabar would print to foo
• *:5>foo to raise the logging level and redirect to foo
• bundle2:5>foo to raise the logging level of the bundle2 category and redirect it to foo
• etc.

[glandium]

Done in 385f6ddf14e3b05239a58d551100699cdd47331e