files: retain a Shift-selected region when switching between buffers

The user did not move the cursor nor intend to move the cursor, so
leave things as they are.

This fixes https://savannah.gnu.org/bugs/?55535.
Indirectly-reported-by: David Lawrence Ramsey <pooka109@gmail.com>

src/files.c

@@ -661,6 +661,9 @@ void switch_to_adjacent_buffer(bool to_next)
 	/* Ensure that the main loop will redraw the help lines. */
 	currmenu = MMOST;
 
+	/* Prevent a possible Shift selection from getting cancelled. */
+	shift_held = TRUE;
+
 	/* Indicate on the status bar where we switched to. */
 	mention_name_and_linecount();
 }