fix git 1.7.7 breakage

* This version of git-annex only works with git 1.7.7 and newer.
  The breakage with old versions is subtle, and affects
  annex.numcopies .gitattributes settings, so be sure to upgrade git
  to 1.7.7. (Debian package now depends on that version.)
* Don't pass absolute paths to git show-attr, as it started following
  symlinks when that's done in 1.7.7. Instead, use relative paths,
  which show-attr only handles 100% correctly in 1.7.7. Closes: #645046

Unfortunatly I can find no way to work with the old and new gits, as
the old had bugs that require absolute paths, while the new doesn't like
them at all. And the behavior of git show-attr in 1.7.7. is the same as
eg, git add of an absolute path to a symlink, so seems entirely
intentional and not likely to change.

debian/control

@@ -29,7 +29,7 @@ Package: git-annex
 Architecture: any
 Section: utils
 Depends: ${misc:Depends}, ${shlibs:Depends},
-	git | git-core,
+	git (>= 1:1.7.7),
 	uuid,
 	rsync,
 	wget | curl,