In the interactive conflcit prompt, make the 'merge' option try the external
merge tool first, and fall back to the internal merge tool if the external
tool fails.

Avoids confusion if users configure an external merge tool and then
attempt to use the 'm' option to launch that tool (instead of using
the 'l' option).


* subversion/svn/conflict-callbacks.c
  (launch_resolver): Return any error from svn_cl__merge_file_externally(),
   instead of suggesting the 'm' option on error.
  (text_conflict_options): Tweak help string for 'm' option accordingly.
  (handle_text_conflict): Try external merge tool first if users picks 'm',
   fall back to internal merge tool if the external tool fails to run.
   Suggest the internal merge tool ('m' option) if the user picks 'l' but
   the external tool fails to run.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1524145 13f79535-47bb-0310-9956-ffa450edef68