@@ -27,6 +27,7 @@ b,branch=     create a new branch from the split subtree
 ignore-joins  ignore prior --rejoin commits
 onto=         try connecting new tree to an existing one
 rejoin        merge the new branch back into HEAD
+clear-cache   reset the subtree mapping cache
  options for 'add', 'merge', and 'pull'
 squash        merge subtree changes as a single commit
 "

@@ -48,6 +49,7 @@ annotate=
 squash=
 message=
 prefix=
+clearcache=
 
 debug () {
 	if test -n "$debug"

@@ -131,6 +133,9 @@ do
 	--no-rejoin)
 		rejoin=
 		;;
+	--clear-cache)
+		clearcache=1
+		;;
 	--ignore-joins)
 		ignore_joins=1
 		;;

@@ -206,9 +211,13 @@ debug "opts: {$*}"
 debug
 
 cache_setup () {
-	cachedir="$GIT_DIR/subtree-cache/$$"
-	rm -rf "$cachedir" ||
-		die "Can't delete old cachedir: $cachedir"
+	cachedir="$GIT_DIR/subtree-cache/$prefix"
+	if test -n "$clearcache"
+	then
+		debug "Clearing cache"
+		rm -rf "$cachedir" ||
+			die "Can't delete old cachedir: $cachedir"
+	fi
 	mkdir -p "$cachedir" ||
 		die "Can't create new cachedir: $cachedir"
 	mkdir -p "$cachedir/notree" ||

@@ -266,6 +275,16 @@ cache_set () {
 	echo "$newrev" >"$cachedir/$oldrev"
 }
 
+cache_set_if_unset () {
+	oldrev="$1"
+	newrev="$2"
+	if test -e "$cachedir/$oldrev"
+	then
+		return
+	fi
+	echo "$newrev" >"$cachedir/$oldrev"
+}
+
 rev_exists () {
 	if git rev-parse "$1" >/dev/null 2>&1
 	then

@@ -375,13 +394,13 @@ find_existing_splits () {
 			then
 				# squash commits refer to a subtree
 				debug "  Squash: $sq from $sub"
-				cache_set "$sq" "$sub"
+				cache_set_if_unset "$sq" "$sub"
 			fi
 			if test -n "$main" -a -n "$sub"
 			then
 				debug "  Prior: $main -> $sub"
-				cache_set $main $sub
-				cache_set $sub $sub
+				cache_set_if_unset $main $sub
+				cache_set_if_unset $sub $sub
 				try_remove_previous "$main"
 				try_remove_previous "$sub"
 			fi

@@ -690,6 +709,8 @@ process_split_commit () {
 		if test -n "$newparents"
 		then
 			cache_set "$rev" "$rev"
+		else
+			cache_set "$rev" ""
 		fi
 		return
 	fi

@@ -787,7 +808,7 @@ cmd_split () {
 			# the 'onto' history is already just the subdir, so
 			# any parent we find there can be used verbatim
 			debug "  cache: $rev"
-			cache_set "$rev" "$rev"
+			cache_set_if_unset "$rev" "$rev"
 		done
 	fi
 

@@ -800,7 +821,7 @@ cmd_split () {
 		git rev-list --topo-order --skip=1 $mainline |
 		while read rev
 		do
-			cache_set "$rev" ""
+			cache_set_if_unset "$rev" ""
 		done || exit $?
 	fi