scap prep auto: Reset mtime of patched files when reapplying patches

Before resetting a mediawiki checkout, collect the mtime and hash of
each file affected by security patches. After resetting and
reapplying security patches, revert the mtime of any patched file
whose contents didn't change. This helps prevent l10n files from
being rebuilt every time scap backport is used in the presence of a
security patch which modifies an l10n source file.