branch: add read_branch_desc() helper function

This will be used by various callers that make use of the branch
description throughout the system, so that if we need to update
the implementation the callers do not have to be modified.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.h

@@ -46,4 +46,9 @@ void remove_branch_state(void);
 #define BRANCH_CONFIG_VERBOSE 01
 extern void install_branch_config(int flag, const char *local, const char *origin, const char *remote);
 
+/*
+ * Read branch description
+ */
+extern int read_branch_desc(struct strbuf *, const char *branch_name);
+
 #endif