Use exit statuses consistent with Shellfu conventions

dotfiles/gittum/bin/remote-edit

 
 die() {
     warn "$@"
-    exit 1
+    exit 3
 }
 
 usage() {
     warn "usage: git $(basename "$0") [-v|--verbose] REMOTE"
-    exit 0
+    exit 2
 }
 
 think() {
     think "remote='$remote'"
     oldurl=$(git remote get-url "$remote") \
      || die "failed to get old URL"
+    test "$newurl" == "$oldurl" && {
+        warn "URL was not changed"
+        return 1
+    }
     think "oldurl='$oldurl'"
     think "EDITOR='$EDITOR'"
     think "VISUAL='$VISUAL'"