Repeated reopening and closing large projects was causing heap
thrashing due to paging stuff in and out of JGit's internal window
cache. This problem persisted even when preopening the repos to get
them in JGit's internal RepositoryCache.
We can simplify the code and avoid heap thrashing by just rebuilding
one project at a time in each thread. This way we can also reuse a
single NoteDbUpdateManager to make the ref update batching behavior
more transparent.
This does not completely eliminate performance issues, particularly on
spinning disk, but as I write this I am rebuilding AOSP with no
particular CPU thrashing issues.
Change-Id: I2250eb01c8e57e690138dc38ee2cf12b8d0b3aa7
f819a4dc7a