Bug 1275613 - Always schedule a 'file_patterns' job when its task definition has changed, r=dustin

Currently, jobs which has 'file_patterns' set will only be run if a file matching one
of those patterns has been modified. This means that unless the task definition of the
job was explicitly included in those 'file_patterns', the job won't run when we modify
it!

I think it is safe to assume that when making changes to a job's task, we want it to be
run no matter what. So let's add the task definition to 'file_patterns' automatically.

Here's a try run which modifies the marionette harness task:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=01e6cf9d8599c64d61ac8ecb5320c1c4209e0a8b

MozReview-Commit-ID: AUSfUFzhiA1

--HG--
extra : rebase_source : de9d66d1d17de90dc83681e51134637616a0652e

taskcluster/taskgraph/kind/legacy.py

@@ -191,6 +191,8 @@ class LegacyKind(base.Kind):
             # of the changed files.
             file_patterns = when.get('file_patterns', None)
             if file_patterns and changed_files:
+                # Always consider changes to the task definition itself
+                file_patterns.append('testing/taskcluster/{task}'.format(task=task['task']))
                 for pattern in file_patterns:
                     for path in changed_files:
                         if mozpackmatch(path, pattern):

testing/taskcluster/mach_commands.py

@@ -311,6 +311,8 @@ class Graph(object):
             # of the changed files.
             file_patterns = when.get('file_patterns', None)
             if file_patterns and changed_files:
+                # Always consider changes to the task definition itself
+                file_patterns.append('testing/taskcluster/{task}'.format(task=task['task']))
                 for pattern in file_patterns:
                     for path in changed_files:
                         if mozpackmatch(path, pattern):