lookupkey absolute path support

lookupkey: Support being given an absolute filename to a file within the
current git repository.

This commit was supported by the NSF-funded DataLad project.

Command/LookupKey.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2013 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -9,6 +9,7 @@ module Command.LookupKey where
 
 import Command
 import Annex.CatFile
+import qualified Git.LsFiles
 
 cmd :: Command
 cmd = notBareRepo $ noCommit $ noMessages $
@@ -18,10 +19,21 @@ cmd = notBareRepo $ noCommit $ noMessages $
 		(batchable run (pure ()))
 
 run :: () -> String -> Annex Bool
-run _ file = do
-	mk <- catKeyFile file
-	case mk of
+run _ file = seekSingleGitFile file >>= \case
+	Nothing -> return False
+	Just file' -> catKeyFile file' >>= \case
 		Just k  -> do
 			liftIO $ putStrLn $ key2file k
 			return True
 		Nothing -> return False
+
+-- To support absolute filenames, pass through git ls-files.
+-- But, this plumbing command does not recurse through directories.
+seekSingleGitFile :: FilePath -> Annex (Maybe FilePath)
+seekSingleGitFile file = do
+	(l, cleanup) <- inRepo (Git.LsFiles.inRepo [file])
+	r <- case l of
+		(f:[]) | takeFileName f == takeFileName file -> return (Just f)
+		_ -> return Nothing
+	void $ liftIO cleanup
+	return r