fix traceback of extdiff after a merge
- files that comes from a different branch are marked as modified
but aren't present in the original manifest
- add a testcase for extdiff and for regular diff
#!/bin/shhg initmkfifo phg serve --stdio < p &P=$!(echo lock; echo addchangegroup; sleep 5) > p &Q=$!sleep 3kill -HUP $Pwaitls .hg