Send the base commit ID for a change to the server when using Git.

When uploading a change to a new enough Review Board server, the base
commit ID is sent along in the request. This ID/revision represents the
upstream commit that the diff (or its parent diff) applies to. It can be
used by the server to communicate with hosting services that otherwise
do not work solely with the content found in the diffs.
Enhance Review Board RBTools (rbt) to support svn tree (or file) copy

RBTools (rbt post) does NOT support svn tree copy (or file/files copy) from
other branches. With this change, rbt notifies user that it has detected
tree/file copy and asks the user if the review should contain files added as
part of svn copy or not.

Testing done:
$ svn cp $svn/branch/dir .
A    dir/a.c
A    dir/b.c
Checked out revision XXXXX.
A         dir

$ svn st
A  +    dir

1. Test using NO svn changelist
$ rbt post
One or more files in your changeset has history scheduled with commit.
Please add option '--svn-show-copies-as-adds=y/n' and re-run

$ rbt post --svn-show-copies-as-adds=y
Review request #ddddd posted.

2. Test using svn changelist
$ rbt post --svn-changelist=tree_copy_1_file

One or more files in your changeset has history scheduled with commit.
Please add option '--svn-show-copies-as-adds=y/n' and re-run

$ rbt post  --svn-changelist=tree_copy_1_file
--svn-show-copies-as-adds=y
Review request #ddddd posted.

Fixes bug 3024
Reviewed at https://reviews.reviewboard.org/r/4279/