patcher: handle missing signature

[xmo-odoo]

Git supports format-patch --no-signature, and that makes the patcher very unhappy as all the content ends up in the third parameter of partition, so the patch data is empty, which is incorrect.

Also:

• might be a good idea to convert windows line endings to unix? Might lead to breakage if we actually want windows line ends but that seems doubtful, apparently the ace widget adds windows line-ends on windows...
• probably handle signature separators with just --\n