Bug 1610886 - Make it easier to run test-verify on try; r=bc

Ensure a minimum of 1 chunk for "per-file" tests like test-verify when run on try, even when
no local file changes are detected. --full is still required.

Differential Revision: https://phabricator.services.mozilla.com/D67694

taskcluster/taskgraph/util/perfile.py

@@ -83,4 +83,11 @@ def perfile_number_of_chunks(is_try, try_task_config, head_repository, head_rev,
                     test_count += 1
 
     chunks = test_count/tests_per_chunk
-    return int(math.ceil(chunks))
+    chunks = int(math.ceil(chunks))
+
+    # Never return 0 chunks on try, so that per-file tests can be pushed to try with
+    # an explicit path, and also so "empty" runs can be checked on try.
+    if is_try and chunks == 0:
+        chunks = 1
+
+    return chunks