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.

debian/changelog

@@ -9,6 +9,9 @@ git-annex (5.20141232) UNRELEASED; urgency=medium
   * Android: Provide a version built with -fPIE -pie to support Android 5.0.
   * sync: Fix an edge case where syncing in a bare repository would try to
     merge and so fail.
+  * 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.
 
  -- Joey Hess <id@joeyh.name>  Fri, 02 Jan 2015 13:35:13 -0400