findkeys: New command, very similar to git-annex find but operating on keys

I've long been asked for `git-annex find --all` or something like that,
but pushed back on it because I feel that the command is analagous to
find(1) and so it would be surprising for it to list keys rather than
files. So instead, add a new findkeys subcommand.

Note that the use of withKeyOptions is rather strange because usually
that is used to fall back to --all rather than listing files, but here
it's made to default to --all like behavior and never list files.

A performance thing that could be improved is that withKeyOptions
always reads and caches location logs. But findkeys with no options does
not need them, so it could be made faster. That caching does speed up
options like --in though. This is really just a subset of a more general
performance thing that --all reads location logs sometimes unncessarily.
Anyway, it needs to read the location log in order to checkDead,
and it seems good that findkeys does skip dead keys.

Also, cleaned up comments on git-annex-find man page asking for --all
option.

Sponsored-by: Dartmouth College's DANDI project

Command/Find.hs

@@ -43,28 +43,26 @@ optParser desc = FindOptions
 	<*> parseBatchOption False
 
 parseFormatOption :: Parser Utility.Format.Format
-parseFormatOption = 
+parseFormatOption = parseFormatOption' "${file}\0"
+
+parseFormatOption' :: String -> Parser Utility.Format.Format
+parseFormatOption' print0format = 
 	option (Utility.Format.gen <$> str)
 		( long "format" <> metavar paramFormat
 		<> help "control format of output"
 		)
-	<|> flag' (Utility.Format.gen "${file}\0")
+	<|> flag' (Utility.Format.gen print0format)
 		( long "print0"
-		<> help "output filenames terminated with nulls"
+		<> help "use nulls to separate output rather than lines"
 		)
 
 seek :: FindOptions -> CommandSeek
 seek o = do
 	unless (isJust (keyOptions o)) $
 		checkNotBareRepo
-	islimited <- limited
-	let seeker = AnnexedFileSeeker
+	seeker <- contentPresentUnlessLimited $ AnnexedFileSeeker
 		{ startAction = start o
-		-- only files with content present are shown, unless
-		-- the user has requested others via a limit
-		, checkContentPresent = if islimited
-			then Nothing
-			else Just True
+		, checkContentPresent = Nothing
 		, usesLocationLog = False
 		}
 	case batchOption o of
@@ -77,6 +75,17 @@ seek o = do
   where
 	ww = WarnUnmatchLsFiles
 
+-- Default to needing content to be present, but if the user specified a
+-- limit, content does not need to be present.
+contentPresentUnlessLimited :: AnnexedFileSeeker -> Annex AnnexedFileSeeker
+contentPresentUnlessLimited s = do
+	islimited <- limited
+	return $ s
+		{ checkContentPresent = if islimited
+			then Nothing
+			else Just True
+		}
+
 start :: FindOptions -> SeekInput -> RawFilePath -> Key -> CommandStart
 start o _ file key = startingCustomOutput key $ do
 	showFormatted (formatOption o) file