avoid git check-ignore overhead on importing known files

isKnownImportLocation does a database lookup and there's an index
to make that lookup fast, so it's probably faster than talking to git
check-ignore. Checking the matcher is faster still.

While before the gitignore check was added it did not need to always
check isknown, now it does, because it's that or the more expensive
notignored. But at least we can skip notignored when a file is known,
which will often be the common case: Importing from a remote that's been
exported to, and/or imported from before, only new files will not be
known, so only those will need to check notignored.

At first, I had this:
	(matches <&&> (isknown <||> notignored)) <||> isknown
Notice that checks isknown every time, whether it matches or not.

So, it's no slower to instead do this:
	isknown <||> (matches <&&> notignored)
That has the benefit that, when it's known, it doesn't need to run
matches, which while faster than isknown, is still going to use some CPU.

And it perhaps more clearly expresses the condition: Any known file is
wanted, otherwise it's down to what matches and is not ignored.

This commit was sponsored by Jack Hill on Patren.

Annex/Import.hs

@@ -643,25 +643,22 @@ getImportableContents r importtreeconfig ci matcher =
 		<*> mapM (filterunwanted dbhandle) (importableHistory ic)
 	
 	wanted dbhandle (loc, (_cid, sz))
-		| ".git" `elem` Posix.splitDirectories (fromImportLocation loc) =
-			pure False
-		| otherwise = wantImport importtreeconfig ci matcher loc sz
-			<||> isKnownImportLocation dbhandle loc
+		| ingitdir = pure False
+		| otherwise =
+			isknown <||> (matches <&&> notignored)
+	  where
+		-- Checks, from least to most expensive.
+		ingitdir = ".git" `elem` Posix.splitDirectories (fromImportLocation loc)
+		matches = matchesImportLocation matcher loc sz
+		isknown = isKnownImportLocation dbhandle loc
+		notignored = notIgnoredImportLocation importtreeconfig ci loc
 
 isKnownImportLocation :: Export.ExportHandle -> ImportLocation -> Annex Bool
 isKnownImportLocation dbhandle loc = liftIO $
 	not . null <$> Export.getExportTreeKey dbhandle loc
 
-{- The matcher is matched relative to the top of the tree of files on the
- - remote, even when importing into a subdirectory. 
- -
- - However, when checking gitignores, the subdirectory is included
- - so it will look at the gitignore file in it.
- -}
-wantImport :: ImportTreeConfig -> CheckGitIgnore -> FileMatcher Annex -> ImportLocation -> ByteSize -> Annex Bool
-wantImport importtreeconfig ci matcher loc sz =
-	checkMatcher' matcher mi mempty
-		<&&> (not <$> checkIgnored ci f)
+matchesImportLocation :: FileMatcher Annex -> ImportLocation -> Integer -> Annex Bool
+matchesImportLocation matcher loc sz = checkMatcher' matcher mi mempty
   where
 	mi = MatchingInfo $ ProvidedInfo
 		{ providedFilePath = fromImportLocation loc
@@ -670,6 +667,10 @@ wantImport importtreeconfig ci matcher loc sz =
 		, providedMimeType = Nothing
 		, providedMimeEncoding = Nothing
 		}
+
+notIgnoredImportLocation :: ImportTreeConfig -> CheckGitIgnore -> ImportLocation -> Annex Bool
+notIgnoredImportLocation importtreeconfig ci loc = not <$> checkIgnored ci f
+  where
 	f = fromRawFilePath $ case importtreeconfig of
 		ImportSubTree dir _ ->
 			getTopFilePath dir P.</> fromImportLocation loc