Update git tarball instead of writing a new one

The archive tarball created by git has a pax header comment referencing
the sha1 of the commit it was created for (see `git get-tar-commit-id`).
This would be lost when moving everything to a new tarball. In the rare
case that the submodule command did not actually create any new tarball
content, this will now leave the original git tarball unmodified.

src/port1.0/portfetch.tcl

@@ -527,20 +527,17 @@ proc portfetch::gitfetch {args} {
         set xz [findBinary xz ${portutil::autoconf::xz_path}]
         # TODO: add dependency on libarchive, if /usr/bin/tar is not bsdtar
         set tar [findBinary bsdtar tar]
-        set tartmp [join [list [mktemp "/tmp/macports.portfetch.${name}.XXXXXXXX"] ".tar"] ""]
         # determine tmppath again in shell, as the real path might be different
         # due to symlinks (/tmp vs. /private/tmp), pass it as MPTOPDIR in
         # environment
         set cmdstring [join [list \
             "MPTOPDIR=\$PWD " \
             "${git.cmd} submodule -q foreach --recursive '" \
             "${git.cmd} archive --format=tar --prefix=\"${git.file_prefix}/\${PWD#\$MPTOPDIR/}/\" \$sha1 " \
-            "| tar -cf ${tartmp} @- @${tardst} " \
-            "&& mv ${tartmp} ${tardst}" \
+            "| tar -uf ${tardst} @-" \
             "' 2>&1"] ""]
         if {[catch {system -W $tmppath $cmdstring} result]} {
             delete $tardst
-            delete $tartmp
             return -code error [msgcat::mc "Git submodule archive creation failed"]
         }
     }