Automatically fetch PRs for rebase if not already merged

[manics]

Rebase an unmerged PR, this will fail if that commit hasn't already been fetched:

$ scc rebase 5460 metadata53
2017-08-22 17:26:48,640 [  scc.rebase] INFO  PR 5460: allow model types to be ignored during object deletion opened by Mark Carroll against develop
2017-08-22 17:26:48,641 [  scc.rebase] INFO  Head: 338362
2017-08-22 17:26:49,047 [  scc.rebase] INFO  Merged: False
Commit 338362bd08634334a2198997bf89c95a029171d7 does not exists in local Git repository. Fetch this remote first: mtbc

Since it is possible to fetch PRs from origin without needing to add a remote scc should automatically fetch pull/<PR>/head:

$ git fetch origin pull/5460/head
remote: Counting objects: 23, done.
remote: Total 23 (delta 19), reused 19 (delta 19), pack-reused 4
Unpacking objects: 100% (23/23), done.
From https://github.com/openmicroscopy/openmicroscopy
 * branch                  refs/pull/5460/head -> FETCH_HEAD

$ scc rebase 5460 metadata53
2017-08-22 17:30:24,671 [  scc.rebase] INFO  PR 5460: allow model types to be ignored during object deletion opened by Mark Carroll against develop
...
# Created local branch rebased/metadata53/delete-ignore-types
# Pushed rebased/metadata53/delete-ignore-types to manics
https://github.com/openmicroscopy/openmicroscopy/pull/5464