merge: add "other" file node in the merge state file
This data is mostly redundant with the "other" changeset node (+ other changeset
file path). However, more data never hurt.
The old format do not store it so this require some dancing to add and remove it
on demand.
# The 9mail to support patchbomb and other email wrappers
[email]
method = /bin/mercurial/9mail