Fix subrepo init failing if defaultbranch not set

subrepo uses -e to ensure that all commands succeed, and fails
immediately if they do not. git config reports a failure if a
setting is not found causing subrepo init to silently fail if
the setting did not exist on the system. Updated to use --default
which returns a default value if the setting does not exist.

Fixes #631

lib/git-subrepo

@@ -211,9 +211,7 @@ command:clone() {
 # `git subrepo init <subdir>` command:
 command:init() {
   command-setup +subdir
-  local default_branch=
-  default_branch=$(git config init.defaultbranch)
-  default_branch=${default_branch:=master}
+  local default_branch=$(git config get --default=master init.defaultbranch)
   local remote=${subrepo_remote:=none}
   local branch=${subrepo_branch:=$default_branch}