Add a new "i" option to the interactive text conflict prompt which
runs the internal merge tool.

So now have:
  "m": use external merge tool if configured, or internal one if not
  "l": use external merge tool
  "i": use internal merge tool

This gives users full control over which merge tool is run.

Only the "m" option is displayed at the initial prompt,
since this is what we expect most people will want to use.
The other options are shown in the long help text selected
by the "s" (or "?") option.

* subversion/svn/cl-conflicts.c
  (text_conflict_options): Add new "i" option.
   Tweak description of "l" option for consistency.
  (handle_text_conflict): Implement the new "i" option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1589709 13f79535-47bb-0310-9956-ffa450edef68