imap-send: cleanup execl() call to use NULL sentinel instead of 0

Some versions of gcc check that calls to the exec() family have the proper
sentinel for variadic calls. This should be (char *) NULL according to the
man page. Although for all other purposes the 0 is equivalent, gcc
nevertheless does emit a warning for 0 and not for NULL. This also makes the
usage consistent throughout git.

The whitespace in function calls throughout imap-send.c has its own style,
so I left it that way.

Signed-off-by: Junio C Hamano <junkio@cox.net>

imap-send.c

@@ -945,7 +945,7 @@ imap_open_store( imap_server_conf_t *srvc )
 				_exit( 127 );
 			close( a[0] );
 			close( a[1] );
-			execl( "/bin/sh", "sh", "-c", srvc->tunnel, 0 );
+			execl( "/bin/sh", "sh", "-c", srvc->tunnel, NULL );
 			_exit( 127 );
 		}