dim: Append to fixup file and allow local branch in dry-mode

If we already have a fixup file for a certain branch, we would override any previous fixups. Just appending to the previous diff (even if already containing the same path) seems to work better.

It's also useful to allow the dry mode to merge a local branch, so we can anticipate if drm-tip will build properly after merging something (without pushing).

Signed-off-by: Lucas De Marchi lucas.demarchi@intel.com