Don't backtrace when current branch is not uploadable.

The backtrace currently occurs when one uses the "--cbr" argument with
the repo upload subcommand if the current branch is not tracking an
upstream branch. There may be other cases that would backtrace as well,
but this is the only one I found so far.

Change-Id: Ie712fbb0ce3e7fe3b72769fca89cc4c0e3d2fce0
1 file changed