Support --ignore-matching-lines option to git log

[dairiki]

Being able to pass a regex to git log's --ignore-matching-lines option could be useful to ignore changes in single-line-value Lektor fields (e.g. the _hidden and _discoverable flags).

Of course --ignore-matching-lines is only supported for git >= 2.30.

If we want to make this configurable on a per-field basis, this will take some refactoring, since, currently we can only cache one git log output per source file. It would be pretty straightforward (module the fact that older gits do not support the --ignore-matching-lines option) to make this a global config option (as was done with follow_renames in 98022c6.