Simplify `rb_git_walker_push_range` and fix a gcc warning.
Simplify `rb_git_walker_push_range` and fix a gcc warning.

Acidburn0zzz/rugged,Acidburn0zzz/rugged,tpickett66/rugged,Acidburn0zzz/rugged,tpickett66/rugged,tpickett66/rugged,saraid/rugged,libgit2/rugged,libgit2/rugged,saraid/rugged,libgit2/rugged,saraid/rugged