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.
When Mercurial accepts more than one revision, they may be specified
individually, or provided as a topologically continuous range,
separated by the ":" character.
The syntax of range notation is [BEGIN]:[END], where BEGIN and END are
revision identifiers. Both BEGIN and END are optional. If BEGIN is not
specified, it defaults to revision number 0. If END is not specified,
it defaults to the tip. The range ":" thus means "all revisions".
If BEGIN is greater than END, revisions are treated in reverse order.
A range acts as a closed interval. This means that a range of 3:5
gives 3, 4 and 5. Similarly, a range of 9:6 gives 9, 8, 7, and 6.