Cleaner instrumentation, and reduce the thread pool size.

Memory pressure might be an issue here.
Greatly simplify the threaded scheduler.  Still isn't reliable, alas.

Non-threaded build passes regression tests.
Prevent a potential race condition.
Suppress verbosity.
Fix an off-by one in the debugging messages.

groff runs to completion now, but the simplificaiuon has flushed
some bug out of cover that affects the regression tests.

Nonthreaded, as usual, the regression tessts pass.
Explain the new scheduler logic.
Properly mutex-lock the tags table.
Change the name of the tag type to conform with the rest of the code's usage.

All regression tests pass.
More correct way to nuke the blob directory.
Revert "add VPATH support to tests/Makefile"

It broke test rebuilding. I can't completely fix it because I don't
understand the template magic David Leonard used.
Stabilize the commit sequencing in the twotag test.
Restore some of David Leaonard's mods to tests/Makefile.
Multithreaded schedular finally looks stable.

The regression-test vug turned out to be due to to two tagged commits
created with the same timestamp. The threaded scheduler can't be anything but
random when that happens. I worked aeound it with a sleep(1) in the test
script and documented the problem.
Make it possible to pass cvs-fast-export options into test productions.
More precise timing.
Typo fix.
Reduce the frequency of cases where the threaded scheduler can cause lossage.
Closer approach to total ordering.
Cleaner handling of statistics.
Discuss the -t option.