Fix diff skipping lines

Backport go-gitea#13154

ParsePatch previously just skipped all lines that start with "+++ " or "--- "
and makes no attempt to see these lines in context.

This PR rewrites ParsePatch to pay attention to context and position
within a patch, ensuring that --- and +++ are only skipped if
appropriate.

This PR also fixes several issues with incomplete files.

Fix https://codeberg.org/Codeberg/Community/issues/308
Fix go-gitea#13153

Signed-off-by: Andrew Thornton <art27@cantab.net>

Author: zeripath