Add more assertions about uncommitted sqlite transactions.

I think we were sometimes leaving half-done sqlite transactions sitting
around for a long time (eg. across sub-calls to .do files).  This
seemed to be okay on Linux, but caused sqlite deadlocks on MacOS.  Most
likely it's not the operating system, but the sqlite version and
journal mode in use.

In any case, the correct thing to do is to actually commit or rollback
transactions, not leave them hanging around.

...unfortunately this doesn't actually fix my MacOS deadlocks, which
makes me rather nervous.

builder.py

@@ -203,6 +203,7 @@ class BuildJob:
         # redo-ifchange, and it might have done it from a different directory
         # than we started it in.  So os.getcwd() might be != REDO_PWD right
         # now.
+        assert(state.is_flushed())
         dn = self.dodir
         newp = os.path.realpath(dn)
         os.environ['REDO_PWD'] = state.relpath(newp, vars.STARTDIR)
@@ -319,6 +320,7 @@ def main(targets, shouldbuildfunc):
     seen = {}
     lock = None
     for t in targets:
+        assert(state.is_flushed())
         if t in seen:
             continue
         seen[t] = 1
@@ -343,6 +345,8 @@ def main(targets, shouldbuildfunc):
             locked.append((f.id,t))
         else:
             BuildJob(t, f, lock, shouldbuildfunc, done).start()
+        state.commit()
+        assert(state.is_flushed())
 
     del lock