tests: fix test-flagprocessor.t flakiness

The test for duplicate flag processors depended on the timestamps
being set in the dirstate to work. If the time between the the
previous failed commit (which would set the timestamp, due to bug
5645) and the attempted commit with the duplicate flag processors was
small enough, it would fail. The failure was caused by a call to
commands.status() early in the commit process. If the dirstate did not
have the timestamp set, it would need to fetch the file content to
compare with. Since two flag processors had been registered, it would
attempted to base64 decode the contents twice, which would of course
fail.

This patch adds a "hg debugrebuilddirstate" to make it deterministic
and also replaces the test case's "hg commit" by simply "hg status",
since that will trigger reading of the contents and thereby use of the
flag processors as noted above.

Differential Revision: https://phab.mercurial-scm.org/D202

tests/test-flagprocessor.t

@@ -161,9 +161,11 b''
   > extension=$TESTDIR/flagprocessorext.py
   > duplicate=$TESTDIR/flagprocessorext.py
   > EOF
-  $ echo 'this should fail' > file
-  $ hg commit -Aqm 'add file'
+  $ hg debugrebuilddirstate
   *** failed to set up extension duplicate: cannot register multiple processors on flag '0x8'.
+  $ hg st 2>&1 | egrep 'cannot register multiple processors|flagprocessorext'
+  *** failed to set up extension duplicate: cannot register multiple processors on flag '0x8'.
+    File "*/tests/flagprocessorext.py", line *, in b64decode (glob)
 
   $ cd ..