push-to-pull-request: Use getopts for macOS compatibility.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/push-to-pull-request

@@ -26,28 +26,34 @@ See also \`reset-to-pull-request\`.
 EOF
 }
 
-args="$(getopt -o '' --long help,merge -n "$0" -- "$@")"
-eval "set -- $args"
-
 merge=
 
-while true; do
-    case "$1" in
-        --help)
+while getopts ":-:" opt; do
+    case $opt in
+        -)
+            case "$OPTARG" in
+                help)
                     usage
                     exit 0
                     ;;
-        --merge)
+                merge)
                     merge=t
-            shift
                     ;;
-        --)
-            shift
-            break
+                *)
+                    echo "Invalid option: --$OPTARG" >&2
+                    exit 1
+                    ;;
+            esac
+            ;;
+        \?)
+            echo "Invalid option: -$OPTARG" >&2
+            exit 1
             ;;
     esac
 done
 
+shift $((OPTIND - 1))
+
 remote_default="$(git config zulip.zulipRemote || echo upstream)"
 pseudo_remote="$(git config zulip.prPseudoRemote || echo)"