Fix resuming a download when using curl.

Noticed a bug; when using curl a workaround for its empty file behavior
overwrote the file content, so it never resumed and always started over.

CHANGELOG

@@ -18,6 +18,7 @@ git-annex (6.20180317) UNRELEASED; urgency=medium
     stat when run on a directory.
   * info: Changed sorting of numcopies stats table, so it's ordered
     by the variance from the desired number of copies.
+  * Fix resuming a download when using curl.
 
  -- Joey Hess <id@joeyh.name>  Mon, 19 Mar 2018 23:13:59 -0400
 

Utility/Url.hs

@@ -295,7 +295,8 @@ download' quiet url file uo = do
 	curl = do
 		-- curl does not create destination file
 		-- if the url happens to be empty, so pre-create.
-		writeFile file ""
+		unlessM (doesFileExist file) $
+			writeFile file ""
 		go "curl" $ headerparams ++ quietopt "-sS" ++
 			[ Param "-f"
 			, Param "-L"