Add options to the interactive file merge which concanenate conflicted
'their' and 'mine' versions.

This is useful when resolving text conflicts in append-only files.

One example is our release signature files, where new PGP signatures are
appended, but never removed or edited. If someone else was quicker with
committing their signature, pick 'm' at the conflict prompt, then pick '12'
during the file merge, then pick 'r' to mark the conflict resolved.

* subversion/svn/file-merge.c
  (merge_chunks): Add new options "12" and "21" which can be used to
    concanenate conflicted chunks in either order.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1491816 13f79535-47bb-0310-9956-ffa450edef68