This speeds up the in-memory version of debugbuilddag that I'm
working on considerably for the case where we want to build just
a 00changelog.i (for discovery tests, for instance).
There are a couple of test changes because node ids in tests
have changed.
The changes to the patch names in test-mq-qdelete.t were required
because they could collide with nodeid abbreviations and newly
actually do (patch "c" collides with id "cafe..." for patch "b").
Overwriting instead of appending to the file removes the [defaults]
section put into the file by run-tests.py.
It also defeats the --inotify option to run-tests.py.
(Nothing was broken yet, but the lack of -d "0 0" cause changeset
hashes to change unexpectedly in a test case I was editing.)
