connect: let callers know if connection is a socket

They might care because they want to do a half-duplex close.
With pipes, that means simply closing the output descriptor;
with a socket, you must actually call shutdown.

Instead of exposing the magic no_fork child_process struct,
let's encapsulate the test in a function.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -865,6 +865,7 @@ extern struct ref *find_ref_by_name(const struct ref *list, const char *name);
 #define CONNECT_VERBOSE       (1u << 0)
 extern struct child_process *git_connect(int fd[2], const char *url, const char *prog, int flags);
 extern int finish_connect(struct child_process *conn);
+extern int git_connection_is_socket(struct child_process *conn);
 extern int path_match(const char *path, int nr, char **match);
 struct extra_have_objects {
 	int nr, alloc;

connect.c

@@ -633,10 +633,15 @@ struct child_process *git_connect(int fd[2], const char *url_orig,
 	return conn;
 }
 
+int git_connection_is_socket(struct child_process *conn)
+{
+	return conn == &no_fork;
+}
+
 int finish_connect(struct child_process *conn)
 {
 	int code;
-	if (!conn || conn == &no_fork)
+	if (!conn || git_connection_is_socket(conn))
 		return 0;
 
 	code = finish_command(conn);