Diffs and overwrite mode should not replace wildcards with full output

[joshtriplett]

Observed in Cargo: when a test fails due to a mismatch in the output, then in addition to providing the new output, snapbox will also show a diff from any wildcards to the full output, and SNAPSHOTS=overwrite will replace the wildcards with the full output. snapbox should not produce or overwrite diffs where the - of the diff is a wildcard and the + of the diff is output matching that wildcard.