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.