Allow using Cobertura XML report in incremental mode

I'm trying to use diff-cover with mypy to enforce any new code to have type annotations. Here's what I use (via tox):

mypy --cobertura-xml-report {envtmpdir} qutebrowser tests {posargs}
diff-cover --fail-under=100 --compare-branch={env:DIFF_BRANCH:origin/{env:GITHUB_BASE_REF:master}} {envtmpdir}/cobertura.xml

That does seem to work fine - however, due to #6076 (cc @mthuurne), it disables incremental mode (causing the runtime of mypy to go up from <1s to ~15s). Note that in my case I don't care about the XML report only containing changed files, because those are the only ones actually relevant for the annotation coverage diff - thus, I think it would be fine to enable the cache for this use-case.

I understand if this is a too specific case, though - I wonder if it'd make sense to have some "enforce annotations for any new code" functionality in mypy itself (perhaps via the incremental mode)?

Start by tracing the mypy entry point for --cobertura-xml-report and the incremental-mode behavior described in issue #6076. Verify the tox example with mypy and diff-cover, and consider the work complete when generating the Cobertura XML report no longer disables incremental mode for this use case.