Apply rtrim() to arc:upstream

Summary: Git spits these out with \n at the end.

Test Plan:
```
>>> orbital ~/devtools/arcanist $ git branch --set-upstream trim master
Branch trim set up to track local branch master.
>>> orbital ~/devtools/arcanist $ arc which --base arc:upstream
RELATIVE COMMIT
If you run 'arc diff', changes between the commit:

    acf7600e6e728395  Temporarily restore apache/license linters

...and the current working copy state will be sent to Differential, because
it is the merge-base of the upstream of the current branch and HEAD, and
matched the rule 'arc:upstream' in your args 'base' configuration.

You can see the exact changes that will be sent by running this command:

    $ git diff acf7600e6e728395..HEAD

These commits will be included in the diff:

    3580555e4b30598f  WIP

MATCHING REVISIONS
These Differential revisions match the changes in this working copy:

    (No revisions match.)

Since there are no revisions in Differential which match this working copy, a
new revision will be created if you run 'arc diff'.
```

Reviewers: brennantaylor

Reviewed By: brennantaylor

CC: aran

Differential Revision: https://secure.phabricator.com/D4913