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 "--"