Rewrite the way git-flow initialized its variables in git-flow and assumed
existence of a valid git repo. Instead, functions gitflow_load_settings()
and gitflow_require_git_repo() have been added that can be called in each
submodule that requires such.

Specifically, git-flow init does NOT use this.
diff --git a/git-flow-feature b/git-flow-feature
index af1041d..8d335c3 100644
--- a/git-flow-feature
+++ b/git-flow-feature
@@ -12,6 +12,8 @@
 # Copyright (c) 2010 by Benedikt Böhm
 #
 
+gitflow_require_git_repo
+gitflow_load_settings
 PREFIX=$(git config --get gitflow.prefix.feature || echo feature/)
 
 usage() {