better key matching with a regexp

Handles keys that are substrings of other keys, as well as pointer files
that contain a newline after the key.

Note that -S does not match regexp, while -G does by default. Docs are
not clear, determined experimentally. The only other difference in
changing to -G is that if a file used to contain the key and changed
in some way, while still containing the key, -G will match and -S would
not. So eg, annex links that git annex fix rewrites will match, and
files that change lock status will match. Which is an improvement anyway.

Sponsored-by: Jochen Bartl on Patreon

doc/bugs/whereused_false_match_on_longer_key.mdwn

@@ -5,3 +5,5 @@ One way this is likely to happen is SHA256 keys with and without extension,
 for the same content. Or WORM keys with similar filenames.
 
 --[[Joey]]
+
+> [[fixed|done]] --[[Joey]]