hack: no need to git fetch in CI

CIs are assumed to do a git fetch and git merge before running tests.
Therefore, no need for a git fetch inside our validate scripts in CI.

If VALIDATE_ORIGIN_BRANCH is set, then git fetch is skipped and
VALIDATE_ORIGIN_BRANCH is used in validate scripts.

Otherwise, behavior is unchanged.

Signed-off-by: Tibor Vass <tibor@docker.com>

Makefile

@@ -15,6 +15,7 @@ export DOCKER_GITCOMMIT
 # against these are used in hack/validate/.validate to check what changed in the PR.
 export VALIDATE_REPO
 export VALIDATE_BRANCH
+export VALIDATE_ORIGIN_BRANCH
 
 # env vars passed through directly to Docker's build scripts
 # to allow things like `make KEEPBUNDLE=1 binary` easily
@@ -54,6 +55,7 @@ DOCKER_ENVS := \
 	-e TIMEOUT \
 	-e VALIDATE_REPO \
 	-e VALIDATE_BRANCH \
+	-e VALIDATE_ORIGIN_BRANCH \
 	-e HTTP_PROXY \
 	-e HTTPS_PROXY \
 	-e NO_PROXY \

hack/validate/.validate

@@ -11,8 +11,11 @@ if [ -z "$VALIDATE_UPSTREAM" ]; then
 
 	VALIDATE_HEAD="$(git rev-parse --verify HEAD)"
 
-	git fetch -q "$VALIDATE_REPO" "refs/heads/$VALIDATE_BRANCH"
-	VALIDATE_UPSTREAM="$(git rev-parse --verify FETCH_HEAD)"
+	if [ -z "$VALIDATE_ORIGIN_BRANCH" ]; then
+		git fetch -q "$VALIDATE_REPO" "refs/heads/$VALIDATE_BRANCH"
+		VALIDATE_ORIGIN_BRANCH=FETCH_HEAD
+	fi
+	VALIDATE_UPSTREAM="$(git rev-parse --verify $VALIDATE_ORIGIN_BRANCH)"
 
 	VALIDATE_COMMIT_LOG="$VALIDATE_UPSTREAM..$VALIDATE_HEAD"
 	VALIDATE_COMMIT_DIFF="$VALIDATE_UPSTREAM...$VALIDATE_HEAD"