hack/vendor/dropreplace: Make the auto-ref pick more reliable

dropreplace now accepts an optional argument that allows to manually
specify a git ref to use for the modules.

It's not required though and if not present it will default to a master
branch from the upstream (preferred) or origin remote.

This is much more reliable than "module@master" which for some reason
tends to not resolve to the actual latest commit on the master branch.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

hack/vendor.sh

@@ -44,7 +44,20 @@ dropreplace() (
 	set -x
 	ref=$1
 	if [ -z "$ref" ]; then
-		ref=master
+		# module@master not always results in the actual latest commit on the
+		# master branch.
+		# Use the actual branch from the upstream or origin remote.
+		for r in "upstream" "origin"; do
+			if git remote get-url "$r" >/dev/null 2>&1; then
+				ref="$r/master"
+				break
+			fi
+		done
+		if [ -z "$ref" ]; then
+			echo "No valid master ref found" >&2
+			exit 1
+		fi
+		echo "Using $ref" >&2
 	fi
 
 	ref=$(git rev-parse "$ref")
@@ -66,7 +79,7 @@ help() {
 	echo "  - tidy: run go mod tidy"
 	echo "  - vendor: run go mod vendor"
 	echo "  - replace: run go mod edit replace for local modules"
-	echo "  - dropreplace: run go mod edit dropreplace for local modules"
+	echo "  - dropreplace [<gitref>]: remove replace rules and update the api and client modules to the provided ref (defaults to upstream/master)"
 	echo "  - all: run tidy && vendor"
 	echo "  - help: show this help"
 }