Make 'svn diff PATH URL' and 'svn diff URL PATH', where exactly two targets
are given and one target is a URL, a new shorthand invocation syntax for
'svn diff --old PATH --new URL' and 'svn diff --old URL --new PATH',
respectively.

These invocations used to error out with a "cannot mix repository and working
copy targets" before. User may intuitively try to diff a working copy to a URL
in the manner allowed by these new shorthands, so this is a usability
enhancement (principle of least surprise).

Suggested by: Alexey Neyman

* subversion/svn/diff-cmd.c
  (svn_cl__diff): Map a 2-target diff invocation where one target is a URL and
   the other target is a path to 'svn diff --old TARGET1 --new TARGET2'.

* subversion/svn/svn.c
  (svn_cl__cmd_table): Update 'svn help diff' output accordingly.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442640 13f79535-47bb-0310-9956-ffa450edef68