Merging multi-source copy/move into the merge-tracking branch

Hyrum and Madan, there were some conflicts on the merge-tracking after
I merged in the multi-source copy/move code from trunk. I think I've
got a reasonable resolution approach here, but was hoping you could
verify with the attached patch. Look for "mergeinfo" in the diff.

Main points:

- mergeinfo member moved from path_driver_cb_baton to path_driver_info

- calculate_target_merge_info() now called in a loop for each copy
sources. I moved the "Setup our PATHS for the path-based editor
drive" loop up above svn_ra_get_commit_editor2() call, which is a
change in fail-fast behavior.