Most tests are in-memory for performance reasons, which uses JGit's
InMemoryRepository. This repo implementation is fairly well tested, not
least because it's been used by Gerrit for years, but it may differ in
various ways from the FileRepository implementation. Most importantly
for the immediate NoteDb future, we want to make sure all tests continue
to work with a brand-new BatchRefUpdate implementation[1]. Running tests
against on-disk repositories is an easy way to increase coverage of the
new code.
This option is still not on by default for performance reasons, and it's
unlikely we'll enable it in CI.
[1] https://git.eclipse.org/r/100771
Change-Id: I319bdaa81a4b21e36d6615c80a04679d1bf7db07
96896652af