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-hotfix b/git-flow-hotfix
index 9bfd058..357eb78 100644
--- a/git-flow-hotfix
+++ b/git-flow-hotfix
@@ -12,6 +12,8 @@
 # Copyright (c) 2010 by Benedikt Böhm
 #
 
+gitflow_require_git_repo
+gitflow_load_settings
 VERSION_PREFIX=$(git config --get gitflow.prefix.versiontag)
 PREFIX=$(git config --get gitflow.prefix.hotfix || echo hotfix/)