tests/test-bad-extension.t

manifestmerge: use dicthelpers.diff and join

This patch improves manifestmerge performance significantly.

In a repository with 170,000 files, the following results were observed on a
clean working directory. Revision '.' adds one file.

hg perfmergecalculate -r .
- before: 0.41 seconds
- after: 0.13 seconds

hg perfmergecalculate -r .^
- before: 0.53 seconds
- after: 0.24 seconds

Comparing against '.' is much faster than comparing against '.^' because with
'.', the wctx and p2 manifest strings have the same identity, so comparisons
are simply pointer equality. With '.^', the strings have different identities
so we need to perform memcmps.

Any operation that uses manifestmerge benefits.
- hg update . goes from 2.04 seconds to 1.75
- hg update .^ goes from 2.52 seconds to 2.25
- hg rebase -r . -d .~6 (involves 4 merges) goes from 11.8 seconds to 10.8

  $ echo 'raise Exception("bit bucket overflow")' > badext.py
  $ abspath=`pwd`/badext.py

  $ echo '[extensions]' >> $HGRCPATH
  $ echo "gpg =" >> $HGRCPATH
  $ echo "hgext.gpg =" >> $HGRCPATH
  $ echo "badext = $abspath" >> $HGRCPATH
  $ echo "badext2 =" >> $HGRCPATH

  $ hg -q help help
  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
  *** failed to import extension badext2: No module named badext2
  hg help [-ec] [TOPIC]
  
  show help for a given topic or a help overview