Before updating a review request or creating a new one, `rbt post`

command validates the diff file and terminates if it does not pass

validation. This prevents the command from creating empty review

requests when the supplied diff file is invalid.

Tested rbt post with:

1. Empty diff (error 219)

2. Diff from a different repository (error 207)

3. Non-diff file as a diff (error 224)

4. Valid diff (success)
using:

1. Git

2. Mercurial
Also tested base_dir with a subversion reporitory.