cli: Flush stdout before fork()ing to run hooks

Without this flush, if stdout is block buffered (which will happen if
it's a pipe or a file, for example) and the hook also writes to
stdout, then notmuch new's output will appear *after* the hook output.

This situation may be a little esoteric, but it's good practice to
flush before you fork anyway.

hooks.c

@@ -50,6 +50,9 @@ notmuch_run_hook (const char *db_path, const char *hook)
 	goto DONE;
     }
 
+    /* Flush any buffered output before forking. */
+    fflush (stdout);
+
     pid = fork();
     if (pid == -1) {
 	fprintf (stderr, "Error: %s hook fork failed: %s\n", hook,