Check git version at runtime, rather than assuming it will be the same as the git version used at build time when running git-checkattr and git-branch remove.

It's ok to probe every time for git-branch remove because that's
run quite rarely. For git-checkattr, it's run only once, when
starting the --batch mode, and so again the overhead is pretty minimal.

This leaves 2 places where the build version is still used.
git merge might be interactive or fail if one skews, and --no-gpg-sign
might not be pased, or might be passed to a git that doesn't understand it
if the other skews. It seems a little expensive to check the git version
each time these are used.

This doesn't seem likely to cause many problems, at least compared with
check-attr hanging on skew.

Git/Version.hs

@@ -7,6 +7,7 @@
 
 module Git.Version (
 	installed,
+	older,
 	normalize,
 	GitVersion,
 ) where
@@ -22,3 +23,8 @@ installed = normalize . extract <$> readProcess "git" ["--version"]
 	extract s = case lines s of
 		[] -> ""
 		(l:_) -> unwords $ drop 2 $ words l
+
+older :: String -> IO Bool
+older n = do
+	v <- installed
+	return $ v < normalize n