patchbomb: respect HGPLAIN when piping --test output to PAGER
It makes easy to use `hg email --test` from another tool.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
created new head
merging a
warning: conflicts during merge.
merging a failed!
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg update -C' to abandon
e7fe8eb3e180+0d24b7662d3e+ tip
<<<<<<< local
something else
=======
something
>>>>>>> other
M a
? a.orig