Teach "git gerrit-push" new --dry-run option

This tells the script to do everything except actually make the final
changes to the local and remote repositories.  It uses the --dry-run
option of "git push".

While at it, initialize the "refspec" variable and comment handling of
the return code from the push.

Change-Id: I0466be76f5c28122dbf9b7f23a7a679703ca6bb8

Utilities/Scripts/git-gerrit-push

 #!/usr/bin/env bash
 
-USAGE="[<remote>] [--no-topic] [--]"
-
+USAGE="[<remote>] [--no-topic] [--dry-run] [--]"
+OPTIONS_SPEC=
 SUBDIRECTORY_OK=Yes
 . "$(git --exec-path)/git-sh-setup"
 
+#-----------------------------------------------------------------------------
+
 remote=''
+refspecs=''
 no_topic=''
+dry_run=''
 
 # Parse the command line options.
 while test $# != 0; do
   case "$1" in
     --no-topic) no_topic=1 ;;
+    --dry-run)  dry_run=--dry-run ;;
     --) shift; break ;;
     -*) usage ;;
     *) test -z "$remote" || usage ; remote="$1" ;;
@@ -44,8 +49,10 @@ fi
 echo "Fetching $remote master"
 fetch_out=$(git fetch "$remote" master 2>&1) || die "$fetch_out"
 
-# Now push the topic branch
+# Push.  Save output and exit code.
 echo "Pushing to $remote"
-push_stdout=$(git push --porcelain "$remote" $refspecs); push_exit=$?
+push_stdout=$(git push --porcelain $dry_run "$remote" $refspecs); push_exit=$?
 echo "$push_stdout"
+
+# Reproduce the push exit code.
 exit $push_exit