refactor!: remove 'head_ref' from cached_task PR route

This means that pull requests will share cached_tasks between one
another. On the surface this seems wrong, but it is useful when you
rebase on PR on top of another one that has already ran. Besides, the
digest hash should already prevent PRs from re-using cached_tasks that
they shouldn't be.

BREAKING CHANGE: changed cached_task pull request index route format
Fixes: #413

Author: ahal

src/taskgraph/util/cached_tasks.py

@@ -7,9 +7,7 @@
 import time
 
 TARGET_CACHE_INDEX = "{cache_prefix}.cache.level-{level}.{type}.{name}.hash.{digest}"
-TARGET_PR_CACHE_INDEX = (
-    "{cache_prefix}.cache.head.{head_ref}.{type}.{name}.hash.{digest}"
-)
+TARGET_PR_CACHE_INDEX = "{cache_prefix}.cache.pr.{type}.{name}.hash.{digest}"
 EXTRA_CACHE_INDEXES = [
     "{cache_prefix}.cache.level-{level}.{type}.{name}.latest",
     "{cache_prefix}.cache.level-{level}.{type}.{name}.pushdate.{build_date_long}",

test/test_util_cached_tasks.py

@@ -84,12 +84,10 @@ def assert_pull_request(task):
                 "test-domain.cache.level-3.cache-type.cache-name.hash.abc",
                 "test-domain.cache.level-2.cache-type.cache-name.hash.abc",
                 "test-domain.cache.level-1.cache-type.cache-name.hash.abc",
-                "test-domain.cache.head.default.cache-type.cache-name.hash.abc",
+                "test-domain.cache.pr.cache-type.cache-name.hash.abc",
             ]
         },
-        "routes": [
-            "index.test-domain.cache.head.default.cache-type.cache-name.hash.abc"
-        ],
+        "routes": ["index.test-domain.cache.pr.cache-type.cache-name.hash.abc"],
     }