Fix diff skipping lines (#13157)

* Fix diff skipping lines

Backport #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 #13153

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

* Add testcase

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

* fix comment

* simplify error handling

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

* never return io.EOF

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