dim: Fallback to re-encoded patch on fail

If the email comes from a source that encodes the mbox body as base64
for whatever reason, the grep would fail. If it fails, fallback to using
git-mailinfo which will re-encode it.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
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