Improve SHA*E extension extraction code

Do not treat parts of the filename that contain punctuation or other
non-alphanumeric characters as extensions. Before, such characters were
filtered out.

Note that in 45308ec78b "foo.ba__________r"
was munged to ".bar" and so incorrectly treated as an extension. That was
fixed by changing the filter order, but not allowing punctuation seems a
better fix.

This assumes that extensions containing punctuation are rare. "_" seems the
most likely character; I used it in ikiwiki "._comment" files. But I can't
recall seeing it anywhere else. It certianly seems that no commonly used
extensions contain punctuation. If git-annex doesn't treat "._comment"
as an extension, it's not likely to break software that expects to see that
extension like some software expects to see .epub or .mp3.

This commit was sponsored by Jack Hill on Patreon.

Backend/Hash.hs

@@ -94,7 +94,7 @@ selectExtension f
 	| otherwise = intercalate "." ("":es)
   where
 	es = filter (not . null) $ reverse $
-		take 2 $ map (filter validInExtension) $
+		take 2 $ filter (all validInExtension) $
 		takeWhile shortenough $
 		reverse $ splitc '.' $ takeExtensions f
 	shortenough e = length e <= 4 -- long enough for "jpeg"