massive v6 add speed/memory improvement

v6 add: Take advantage of improved SIGPIPE handler in git 2.5 to speed up
the clean filter by not reading the file content from the pipe. This also
avoids git buffering the whole file content in memory.

When built with an older git, still consumes stdin. If built with a newer
git and used with an older one, it breaks, but that's acceptable --
checking the git version every time would make repeated smudge runs slow.

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

Command/Smudge.hs

@@ -16,6 +16,7 @@ import Annex.Ingest
 import Annex.CatFile
 import Logs.Location
 import qualified Database.Keys
+import qualified Git.BuildVersion
 import Git.FilePath
 import Backend
 
@@ -68,7 +69,7 @@ smudge file = do
 
 -- Clean filter is fed file content on stdin, decides if a file
 -- should be stored in the annex, and outputs a pointer to its
--- injested content.
+-- injested content if so. Otherwise, the original content.
 clean :: FilePath -> CommandStart
 clean file = do
 	b <- liftIO $ B.hGetContents stdin
@@ -76,10 +77,18 @@ clean file = do
 		then liftIO $ B.hPut stdout b
 		else ifM (shouldAnnex file)
 			( do
-				-- Even though we ingest the actual file,
-				-- and not stdin, we need to consume all
-				-- stdin, or git will get annoyed.
-				B.length b `seq` return ()
+				-- Before git 2.5, failing to consume all
+				-- stdin here would cause a SIGPIPE and
+				-- crash it.
+				-- Newer git catches the signal and
+				-- stops sending, which is much faster.
+				-- (Also, git seems to forget to free memory
+				-- when sending the file, so the less we
+				-- let it send, the less memory it will
+				-- waste.)
+				if Git.BuildVersion.older "2.5"
+					then B.length b `seq` return ()
+					else liftIO $ hClose stdin
 				-- Look up the backend that was used
 				-- for this file before, so that when
 				-- git re-cleans a file its backend does