please.sh upgrade msys2-runtime: fix logic to determine up-to-date-ness

The `please.sh` script has a rather simple logic to determine whether a
new package should be built or not: comparing the timestamps of the
latest commits of the `msys2-runtime` repository and of the
`msys2-runtime/` subdirectory of the `MSYS2-packages` repository.

However, what we need to look for is a commit in `MSYS2-packages` that
changed either `pkgrel` or `pkgver` in `msys2-runtime/PKGBUILD`.

Let's do that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

please.sh

@@ -2743,7 +2743,7 @@ upgrade () { # [--directory=<artifacts-directory>] [--only-mingw] [--no-build] [
 			 msys2_runtime_mtime=$(git log -1 --format=%ct \
 				git-for-windows/main --) &&
 			 msys2_package_mtime=$(git -C ../.. log -1 \
-				--format=%ct -- .) &&
+				--format=%ct -G'pkg(rel|ver)' -- PKGBUILD) &&
 			 test $msys2_runtime_mtime -gt $msys2_package_mtime
 			 ;;
 		 esac ||