use --no-abbrev instead of --abbrev=40

This avoids hardcoding the sha size, so when git uses sha256, it will
output the full sha256 and not a truncation to 40 characters.

I reviewed git's history, and while there have been some
bugs with commands not supporting --no-abbrev (eg git diff --no-index
--no-abbrev was broken in git 2.1), none of the commands git-annex
uses will be impacted by those old bugs.

Command/Log.hs

@@ -215,7 +215,7 @@ getGitLog fs os = do
 		, Param "-z"
 		, Param "--pretty=format:%ct"
 		, Param "--raw"
-		, Param "--abbrev=40"
+		, Param "--no-abbrev"
 		] ++ os ++
 		[ Param $ Git.fromRef Annex.Branch.fullname
 		, Param "--"

Database/Keys.hs

@@ -260,7 +260,7 @@ reconcileStaged qh = do
 		, Param "--cached"
 		, Param "--raw"
 		, Param "-z"
-		, Param "--abbrev=40"
+		, Param "--no-abbrev"
 		-- Optimization: Only find pointer files. This is not
 		-- perfect. A file could start with this and not be a
 		-- pointer file. And a pointer file that is replaced with

Git/DiffTree.hs

@@ -77,14 +77,14 @@ diffFiles = getdiff (Param "diff-files")
  - is adjusted to be the same as diff-tree --raw._-}
 diffLog :: [CommandParam] -> Repo -> IO ([DiffTreeItem], IO Bool)
 diffLog params = getdiff (Param "log")
-	(Param "-n1" : Param "--abbrev=40" : Param "--pretty=format:" : params)
+	(Param "-n1" : Param "--no-abbrev" : Param "--pretty=format:" : params)
 
 {- Uses git show to get the changes made by a commit.
  -
  - Does not support merge commits, and will fail on them. -}
 commitDiff :: Sha -> Repo -> IO ([DiffTreeItem], IO Bool)
 commitDiff ref = getdiff (Param "show")
-	[ Param "--abbrev=40", Param "--pretty=", Param "--raw", Param (fromRef ref) ]
+	[ Param "--no-abbrev", Param "--pretty=", Param "--raw", Param (fromRef ref) ]
 
 getdiff :: CommandParam -> [CommandParam] -> Repo -> IO ([DiffTreeItem], IO Bool)
 getdiff command params repo = do