Make "make update-packages" idempotent

If the user runs "make update-packages" without bumping BRANCH, then it
isn't possible to copy packages from the old location to the new one
(because the two locations are the same).  So just skip that step.

Sponsored by:		ConnectWise
PR:			295085
MFC after:		1 week
Reviewed by:		ivy, emaste
Differential Revision:	https://reviews.freebsd.org/D56872

Makefile.inc1

@@ -2128,7 +2128,7 @@ real-packages:	stage-packages create-packages sign-packages .PHONY
 
 real-update-packages: stage-packages .PHONY
 	${_+_}${MAKE} -C ${.CURDIR} PKG_VERSION=${PKG_VERSION} create-packages
-.if defined(PKG_VERSION_FROM_DIR)
+.if defined(PKG_VERSION_FROM_DIR) && ${PKG_VERSION} != ${PKG_VERSION_FROM}
 	@echo "==> Checking for new packages (comparing ${PKG_VERSION} to ${PKG_VERSION_FROM})"
 	@for pkg in ${PKG_VERSION_FROM_DIR}/${PKG_NAME_PREFIX}-*; do \
 	  pkgname=$$(${PKG_CMD} query -F $${pkg} '%n' | sed 's/${PKG_NAME_PREFIX}-\(.*\)/\1/') ; \
@@ -2146,6 +2146,8 @@ real-update-packages: stage-packages .PHONY
 	    echo "==> New package $${newpkgname}" ; \
 	  fi ; \
 	done
+.elif defined(PKG_VERSION_FROM_DIR)
+	@echo "==> Package version ${PKG_VERSION} unchanged, keeping old package set"
 .else
 	@echo "==> Bootstrapping repository, not checking for new packages"
 .endif