Support older versions of git for subrep init

Older versions of git require git config --get instead of
git config get. Newer versions support both methods so use the
most widely supported.

Fixes #631

lib/git-subrepo

@@ -211,7 +211,8 @@ command:clone() {
 # `git subrepo init <subdir>` command:
 command:init() {
   command-setup +subdir
-  local default_branch=$(git config get --default=master init.defaultbranch)
+  local default_branch=
+  default_branch=$(git config --get --default=master init.defaultbranch)
   local remote=${subrepo_remote:=none}
   local branch=${subrepo_branch:=$default_branch}