export: clobber files with -o (bc) (issue3652)

This violated user expectation. Updated the code to clobber files, but
preserve the behavior of appending multiple patches requested in a
single export. Includes tests.
parsedate: understand "now" as a shortcut for the current time

--HG--
extra : rebase_source : a2906f188cb708ee3983ac75551256c730b0cef9
outgoing: fix possible filtering crash in outgoing (issue3814)

If there is no outgoiing changesets but we have filtered revision in outgoing.excluded
We run into a filtering related crash. The excluded revision should not be there
in the first place but discovery need cleanup in default, not stable.

--HG--
extra : rebase_source : 304375a4d37ad90582f5b5e9f324dd65c49b95f9
test: display used python hash seed

We keep using a random seed for each run, but we "compute" it ourself to be able
to reproduce a failed test run.