Review Board could not handle added or deleted empty files in SVN, Mercurial,

and Perforce diffs since the diff output from svn diff/hg diff did not

provide enough context on empty files. This change adds extra processing in

RBTools to generate a diff with extra information on any added or deleted empty

files, if the Review Board server has the empty_files capability for the

SCM tool in question.
The original SVN diff output of an added/deleted empty file was:
Index: foo
===================================================================


After this change, the SVN diff of an added empty file becomes:
Index: foo\t(added)
===================================================================
--- foo\t(<base_revision>)
+++ foo\t(<tip_revision>)


For Mercurial, the diff of an added empty file is now:
diff -r <base_revision> -r <tip_revision> foo
--- /dev/null\tThu Jan 01 00:00:00 1970 +0000
+++ b/foo\t<date>


For Perforce, the diff of an added empty file is:
==== //depot/foo#0 ==A== //depot/foo ====

Ran rbt post on a RB server with the empty_files capabilities with different

SVN, Mercurial, and Perforce diffs, and verified that the modified diffs were

as expected:

- Diff with 1 added empty file

- Diff with 1 added empty file, 1 added non-empty file

- Diff with 1 deleted empty file

- Diff with 1 deleted empty file, 1 deleted non-empty file

- Diff with 1 added empty file, 1 deleted empty file

- Diff with 1 added empty file, 1 added non-empty file, 1 deleted empty file,

  1 deleted non-empty file, 1 modified file
Ran rbt post on a RB server without the empty_files capabilities, and

verified that the generated diffs did not include any extra information on

added/deleted empty files. These added/deleted empty files were not included in

the review requests.
Ran unit tests. More testing was done in /r/5785 as well.