Fix #2915

Add forward slash cleanup of the app name for the git-upload-pack
command. Prevents is_valid_app_name from removing the application folder
on `git fetch`.

plugins/git/functions

@@ -153,7 +153,7 @@ git_build() {
 git_upload_pack_cmd() {
   declare desc="executes git-upload-pack"
   local cmd="git-upload-pack"
-  local APP="$(echo "$2" | perl -pe 's/(?<!\\)'\''//g' | sed 's/\\'\''/'\''/g')"
+  local APP="$(echo "$2" | perl -pe 's/(?<!\\)'\''//g' | sed 's/\\'\''/'\''/g' | sed 's/^\///g')"
   is_valid_app_name "$APP"
   plugn trigger git-pre-pull "$APP"
   cat | git-upload-pack "$DOKKU_ROOT/$APP"