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/Remote/Remove.hs

@@ -13,15 +13,17 @@ import Common
 import Git
 import Git.Types
 import qualified Git.Command
-import qualified Git.BuildVersion
+import qualified Git.Version
 
 remove :: RemoteName -> Repo -> IO ()
-remove remotename = Git.Command.run
-	[ Param "remote"
-	-- name of this subcommand changed
-	, Param $
-		if Git.BuildVersion.older "1.8.0"
-			then "rm"
-			else "remove"
-	, Param remotename
-	]
+remove remotename r = do
+	old <- Git.Version.older "1.8.0"
+	Git.Command.run
+		[ Param "remote"
+		-- name of this subcommand changed
+		, Param $
+			if old
+				then "rm"
+				else "remove"
+		, Param remotename
+		] r