Make git-clone respect branch.autosetuprebase

When git-clone creates an initial branch it was not checking the
branch.autosetuprebase configuration option (which may exist in
~/.gitconfig).  Refactor the code used by "git branch" to create
a new branch, and use it instead of the insufficiently duplicated code
in builtin-clone.

Changes are partly, and the test is mostly, based on the previous work by
Pat Notz.

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

branch.h

@@ -21,4 +21,11 @@ void create_branch(const char *head, const char *name, const char *start_name,
  */
 void remove_branch_state(void);
 
+/*
+ * Configure local branch "local" to merge remote branch "remote"
+ * taken from origin "origin".
+ */
+#define BRANCH_CONFIG_VERBOSE 01
+extern void install_branch_config(int flag, const char *local, const char *origin, const char *remote);
+
 #endif