`rbt post` command uses API's DiffValidation endpoint before upload

Review Request #6407 — Created Oct. 6, 2014 and submitted — Latest diff uploaded

Information

RBTools
master
a288b2c...

Reviewers

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.