Add anchors to the regexes for fallback CQ group.

The fallback CQ group seems to be triggering against the M86 branch,
possibly because the regexes intended to match against branches with
numbers with fewer digits match the prefix of refs/branch-heads/4240.

Bug: 1121272
Change-Id: I30d68fde2c27b523c29811ece6fa1b3f64a45d75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372893
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801093}

infra/config/generated/commit-queue.cfg

@@ -2548,25 +2548,25 @@ config_groups {
     url: "https://chromium-review.googlesource.com"
     projects {
       name: "chromium/src"
-      ref_regexp: "refs/branch-heads/.*\\D.*"
-      ref_regexp: "refs/branch-heads/[1-9]\\d{0,2}"
-      ref_regexp: "refs/branch-heads/[1-3]\\d{3}"
-      ref_regexp: "refs/branch-heads/40\\d{2}"
-      ref_regexp: "refs/branch-heads/410[0-2]"
-      ref_regexp: "refs/branch-heads/410[4-9]"
-      ref_regexp: "refs/branch-heads/41[1-3]\\d"
-      ref_regexp: "refs/branch-heads/414[0-6]"
-      ref_regexp: "refs/branch-heads/414[8-9]"
-      ref_regexp: "refs/branch-heads/41[5-7]\\d"
-      ref_regexp: "refs/branch-heads/418[0-2]"
-      ref_regexp: "refs/branch-heads/418[4-9]"
-      ref_regexp: "refs/branch-heads/419\\d"
-      ref_regexp: "refs/branch-heads/42[0-3]\\d"
-      ref_regexp: "refs/branch-heads/424[1-9]"
-      ref_regexp: "refs/branch-heads/42[5-9]\\d"
-      ref_regexp: "refs/branch-heads/4[3-9]\\d{2}"
-      ref_regexp: "refs/branch-heads/[5-9]\\d{3}"
-      ref_regexp: "refs/branch-heads/[1-9]\\d{4,}"
+      ref_regexp: "^refs/branch-heads/.*\\D.*$"
+      ref_regexp: "^refs/branch-heads/[1-9]\\d{0,2}$"
+      ref_regexp: "^refs/branch-heads/[1-3]\\d{3}$"
+      ref_regexp: "^refs/branch-heads/40\\d{2}$"
+      ref_regexp: "^refs/branch-heads/410[0-2]$"
+      ref_regexp: "^refs/branch-heads/410[4-9]$"
+      ref_regexp: "^refs/branch-heads/41[1-3]\\d$"
+      ref_regexp: "^refs/branch-heads/414[0-6]$"
+      ref_regexp: "^refs/branch-heads/414[8-9]$"
+      ref_regexp: "^refs/branch-heads/41[5-7]\\d$"
+      ref_regexp: "^refs/branch-heads/418[0-2]$"
+      ref_regexp: "^refs/branch-heads/418[4-9]$"
+      ref_regexp: "^refs/branch-heads/419\\d$"
+      ref_regexp: "^refs/branch-heads/42[0-3]\\d$"
+      ref_regexp: "^refs/branch-heads/424[1-9]$"
+      ref_regexp: "^refs/branch-heads/42[5-9]\\d$"
+      ref_regexp: "^refs/branch-heads/4[3-9]\\d{2}$"
+      ref_regexp: "^refs/branch-heads/[5-9]\\d{3}$"
+      ref_regexp: "^refs/branch-heads/[1-9]\\d{4,}$"
     }
   }
   verifiers {

infra/config/subprojects/chromium/master-only/fallback-cq.star

@@ -148,9 +148,9 @@ luci.cq_group(
         repo = "https://chromium.googlesource.com/chromium/src",
         refs = (
             # \D - non-digit, match any branch that is not entirely numeric
-            [r"refs/branch-heads/.*\D.*"] +
+            [r"^refs/branch-heads/.*\D.*$"] +
             [
-                "refs/branch-heads/" + regex
+                "^refs/branch-heads/{}$".format(regex)
                 for regex in _get_fallback_branch_number_regexes()
             ]
         ),