Problem:

While the common workflow is to post committed changes, some people have workflows where they want to post changes that are uncommitted, and have to resort to generating a diff manually and posting it using rbt post --diff-filename=. 
This work would add an --uncommitted flag to rbt post that would generate diffs for uncommitted changes instead.
Approach:

Adding can_diff_committed capability to all clients, and let git set that to True to support it.

This revision simulate the behavior of svn, where we specify the tip to be --rbtools-working-copy so that git client know to remove the revision range out of the git commands.

In addition, to support this flag, --update, --diff-filename, and --guess-fields can't coexists, so the error is thrown when the user specifies those commands along with --uncommitted.

In order to view staged but uncommitted files, we need to add --cached to the git diff command.

Write an extra unit test to check that the git post --uncommitted works.

Verify that it throws error when --uncommitted and other options like --diff-filename,