improve rebase when running canvas_update script

no longer forced to switch branches, just rebase master on top
of current branch.

flag = none
closes: DE-363

Test:
- Jenkins docker build passes
- run canvas_update locally with local branch

Change-Id: I6fc7c23d6b18f44a2824a717006cac5974e79307
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/252029
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Aaron Ogata <aogata@instructure.com>
QA-Review: Kyle Rosenbaum <krosenbaum@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>

script/canvas_update

@@ -27,7 +27,6 @@ function update_canvas {
 
   if [[ -z "$SKIP_CODE" ]] ; then
     update_plugins
-    checkout_master_canvas
     rebase_canvas
   fi
 

script/common.sh

@@ -90,7 +90,7 @@ function checkout_master_canvas {
 
 function rebase_canvas {
   echo_console_and_log "  Rebasing canvas-lms on HEAD ..."
-  git pull --rebase >>"$LOG" 2>&1
+  git pull --rebase origin master >>"$LOG" 2>&1
 }
 
 function bundle_install {