Replace custom supports_empty_files by git-style diff
Replace custom supports_empty_files by git-style diff

Previous work-around for support of empty files will collide with Mercurial's
capability of generating git-style diffs. If Mercurial generates a diff with
'-g' it also contain any empty file. So there is no need to handle thse data by
rbtools itself.

Otherwise "rbt diff" will generate something like this duplicate entry:

diff --git a/bla b/bla
new file mode 100644
diff -r 9264e470c193 -r bac4e0592ecc bla
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/bla       Wed Nov 29 15:07:51 2017 +0100

Testing Done:
Created a diff before applying this patch I saw that it has a duplicated enty.
Applied this patch and saw no duplicated entry and no broken "rename from
/dev/null" with a date in diff-viewer of reviewboard.

Bugs closed: 4600

Reviewed at https://reviews.reviewboard.org/r/9412/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools