viewdiff: assume diffstat and diff order are identical
For non-malicious messages, we can assume the diffstat and actual
diff appear in the same order. Thus we can store {-long_paths} as
an arrayref and only compare the first element when we encounter
a truncated path.
This should make HTML rendering stable when there's basename
conflicts in message such as
https://lore.kernel.org/backports/
1393202754-12919-13-git-send-email-hauke@hauke-m.de/
This diffstat anchor linkification can still be defeated by
users who make actual path names beginning with "...", but we
won't waste CPU cycles on it, either.