submodule--helper: move config-sanitizing to submodule.c

These functions should be used by any code which spawns a
submodule process, which may happen in submodule.c (e.g.,
for spawning fetch). Let's move them there and make them
public so that submodule--helper can continue to use them.

Since they're now public, let's also provide a basic overview
of their intended use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.h

@@ -43,4 +43,20 @@ int find_unpushed_submodules(unsigned char new_sha1[20], const char *remotes_nam
 int push_unpushed_submodules(unsigned char new_sha1[20], const char *remotes_name);
 void connect_work_tree_and_git_dir(const char *work_tree, const char *git_dir);
 
+/*
+ * This function is intended as a callback for use with
+ * git_config_from_parameters(). It ignores any config options which
+ * are not suitable for passing along to a submodule, and accumulates the rest
+ * in "data", which must be a pointer to a strbuf. The end result can
+ * be put into $GIT_CONFIG_PARAMETERS for passing to a sub-process.
+ */
+int sanitize_submodule_config(const char *var, const char *value, void *data);
+
+/*
+ * Prepare the "env_array" parameter of a "struct child_process" for executing
+ * a submodule by clearing any repo-specific envirionment variables, but
+ * retaining any config approved by sanitize_submodule_config().
+ */
+void prepare_submodule_repo_env(struct argv_array *out);
+
 #endif