Add possibility to have 2 repositories instead of 1 in diff/diff_summary functions

[jfthuong]

svn diff normally allows to have 2 urls or targets: an old one and a new one.

diff (di): Display local changes or differences between two revisions or paths.
usage: 1. diff
       2. diff [-c M | -r N[:M]] [TARGET[@REV]...]
       3. diff [-r N[:M]] --old=OLD-TGT[@OLDREV] [--new=NEW-TGT[@NEWREV]] \
               [PATH...]
       4. diff OLD-URL[@OLDREV] NEW-URL[@NEWREV]
       5. diff OLD-URL[@OLDREV] NEW-PATH[@NEWREV]
       6. diff OLD-PATH[@OLDREV] NEW-URL[@NEWREV]

It would be nice to have an optional arguments in functions diff and diff_summary to have a new target.