reduce ressource consumption

* git gc --auto should be enough for the rebase

Do not waste ressources on doing a complete aggressive garbage collection run
Additionally git gc gets run by svn2git internally too

svnmigrate.sh

@@ -14,8 +14,8 @@ function migrate() {
 }
 
 function clean() {
-    # clean up git tree thoroughly
-    /usr/bin/git gc --aggressive
+    # clean up git tree
+    /usr/bin/git gc --auto
 }
 
 function push() {