Fix graphing test

Allow `cylc graph` to print plain graph to standard output if the output
file argument is a `-`.

Use single `perl` command to parse graphing result, instead of 2 `sed`
and a `perl`.
Fix blank entry being interpreted as 0 offset.
Add test to ensure ongoing behaviour.
Removed some unnecessary graph update code.
Merge pull request #1167 from arjclark/fix_explicit_null_final_cp

cylc config: Fix explicit null final cycle being interpreted as 0 offset
Merge pull request #1168 from hjoliver/graph-cmd.decruft

Removed some unnecessary graph update code.
Fix syntax that will break in new versions of bash
Merge pull request #1165 from matthewrmshin/fix-graphing-test

Fix graphing test