Bug 1245953 - Support for only running tasks when certain files change; r=garndt

Firefox's automation currently tends to run all the jobs all the time.
It is wasteful to do this. For example, running ESLint when the commit
only changes a .cpp file adds no value.

This commit adds support for only running tasks when certain files
change. The new-style tasks introduced by the previous commit have been
taught a "when" dictionary property that defines conditions that should
hold for the task to be executed. We define a "file_patterns" list that
defines lists of mozpack path matching expressions that will be matched
against the set of files changed by the changesets relevant to the
changeset being built. The eslint task has been updated to only run if
files related to it change.

Because conditions may not be accurate, we add a CLI argument to ignore
conditions and force all would-be-filtered tasks to run.

MozReview-Commit-ID: 3OeBSKAQAeg

testing/taskcluster/taskcluster_graph/commit_parser.py

@@ -350,6 +350,7 @@ def parse_commit(message, jobs):
             # TODO support declaring a different build type
             'build_type': name,
             'interactive': args.interactive,
+            'when': task.get('when', {})
         })
 
     # Times that test jobs will be scheduled