Support newlines in filenames.

Work around git cat-file --batch's protocol not supporting newlines by
running git cat-file not batched and passing the filename as a
parameter.

Of course this is quite a lot less efficient, especially because it
currently runs it multiple times to query for different pieces of
information.

Also, it has subtly different behavior when the batch process was
started and then some changes were made, in which case the batch process
sees the old index but this workaround sees the current index. Since
that batch behavior is mostly a problem that affects the assistant and has
to be worked around in it, I think I can get away with this difference.

I don't know of any other problems with newlines in filenames, everything
else in git I can think of supports -z. And git-annex's json output
supports newlines in filenames so downstream parsers from git-annex will be ok.
git-annex commands that use --batch themselves don't support newlines
in input filenames; using --json --batch is currently a way around that
problem.

This commit was sponsored by Ewen McNeill on Patreon.

Utility/PartialPrelude.hs

@@ -38,11 +38,8 @@ last = Prelude.last
 
 {- Attempts to read a value from a String.
  -
- - Ignores leading/trailing whitespace, and throws away any trailing
- - text after the part that can be read.
- -
- - readMaybe is available in Text.Read in new versions of GHC,
- - but that one requires the entire string to be consumed.
+ - Unlike Text.Read.readMaybe, this ignores leading/trailing whitespace,
+ - and throws away any trailing text after the part that can be read.
  -}
 readish :: Read a => String -> Maybe a
 readish s = case reads s of