tests/test-extdiff.out

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

adding a
making snapshot of 0 files from rev 000000000000
making snapshot of 1 files from working dir
diff -Nr a.000000000000/a a/a
0a1
> a
making snapshot of 0 files from rev 000000000000
making snapshot of 1 files from working dir
diffing a.000000000000 a
hg falabala [OPT]... [FILE]...

use 'echo' to diff repository (or selected files)

            Show differences between revisions for the specified
            files, using the 'echo' program.

            When two revision arguments are given, then changes are
            shown between those revisions. If only one revision is
            specified then that revision is compared to the working
            directory, and, when no revisions are specified, the
            working directory files are compared to its parent.

options:

 -o --option   pass option to comparison program
 -r --rev      revision
 -I --include  include names matching the given patterns
 -X --exclude  exclude names matching the given patterns
making snapshot of 1 files from rev e27a2475d60a
making snapshot of 1 files from rev 5e49ec8d3f05
diffing a.e27a2475d60a a.5e49ec8d3f05
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
making snapshot of 1 files from rev 5e49ec8d3f05
making snapshot of 1 files from working dir
diffing a.5e49ec8d3f05 a
diff-like tools yield a non-zero exit code