Resolves issue #785:
* merge-cmd.c (svn_cl__merge): If no target path was specified, try to
infer it from the source path(s).
* main.c (svn_cl__cmd_table): Document the new use case in the output
of 'svn help merge'. Also, refer to arguments as two "sources" and
a wc "target". [rewritten by sussman]
* merge_tests.py (merge_with_implicit_target): New test function.
Merges a file, specifying a revision range and source file/URL but
no target path. [docstring tweaked by sussman; also move #1176
docstring into issue comment, so docstring fits on one line during run.]
* ch04.xml: Explain the conditions under which 'svn merge' can infer a
target path. [tweaked by sussman]
* ch08.xml: Document the new use case in the reference section for
'svn merge', copying from 'svn help merge' text. Add an example.
[tweaked by sussman]
