Fix crash in error handler.

Fix crash in error handler.

fatal: ambiguous argument 'c97qk-8i9sb-nmca4v1scg9910o': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
bundler: failed to load command: ./script/crunch-dispatch.rb (./script/crunch-dispatch.rb)
NameError: undefined local variable or method `job' for #<CrunchDispatch:0x000000000534bdb8>
  /var/www/arvados-api/current/lib/crunch_dispatch.rb:321:in `tag_commit'

No issue #

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>