[Fix #67] Rename git-branch to git-current-branch

There is a name collision in Git versions older than 1.6.0, which
install the git-* commands in $PATH.

plugins/git/functions/git-hub

@@ -16,7 +16,7 @@ url=$(
 branches=($(
   git branch -r | sed -e "/${remote}\/HEAD -> ${remote}/d" -e "s/^[[:space:]]*${remote}\///g"
 ))
-current_branch="$(git-branch)"
+current_branch="$(git-current-branch)"
 branch="${2:-master}"
 file="$3"