ci: cancel stale pull request workflow runs

The CI workflow groups all runs by commit hash using
`group: ${{ github.sha }}`.  This means every push to a pull
request starts a separate workflow run, and all workflows
triggered by the same commit share the same concurrency group.

With this change, pull request runs are grouped by pull request
number instead of commit hash, and runs superseded by a newer
push are canceled.  The concurrency group becomes
`${{ github.workflow }}-${{ github.event.pull_request.number ||
github.sha }}` and `cancel-in-progress` is set to true for
pull request events.

For pull request events, the group is `<workflow>-<pull-request-number>`
(e.g., "main-workflow-42").  If you push a new commit to an
existing pull request before the CI working on it finishes, the
new request will be placed in the same group and cancel the
currently running run.

For non-pull-request events, the group is `${{ github.workflow }}-${{
github.sha }}` and `cancel-in-progress` defaults to false, so
there is no regression in behavior.

Note that the previous configuration used `group: ${{ github.sha }}`,
which meant all workflows sharing the same commit hash were in the
same group.  The new configuration includes the workflow name in
the group, so each workflow has its own concurrency group per
commit/PR.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -5,18 +5,20 @@ on: [push, pull_request]
 env:
   DEVELOPER: 1
 
-# If more than one workflow run is triggered for the very same commit hash
+# For pull requests, only the latest workflow run is allowed to proceed.
-# (which happens when multiple branches pointing to the same commit), only
+# Older runs are canceled when a new revision is pushed.
-# the first one is allowed to run, the second will be kept in the "queued"
-# state. This allows a successful completion of the first run to be reused
-# in the second run via the `skip-if-redundant` logic in the `config` job.
 #
-# The only caveat is that if a workflow run is triggered for the same commit
+# For pushes, if more than one workflow run is triggered for the very same
-# hash that another run is already being held, that latter run will be
+# commit hash (which happens when multiple branches point to the same commit),
-# canceled. For more details about the `concurrency` attribute, see:
+# only the first one is allowed to run. This allows a successful completion of
+# the first run to be reused in the second run via the `skip-if-redundant`
+# logic in the `config` job.
+#
+# For more details about the `concurrency` attribute, see:
 # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
 concurrency:
-  group: ${{ github.sha }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   ci-config: