rebase: replace incorrect logical negation by correct bitwise one

In bff014dac7 (builtin rebase: support the `verbose` and `diffstat`
options, 2018-09-04), we added a line that wanted to remove the
`REBASE_DIFFSTAT` bit from the flags, but it used an incorrect negation.

Found by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -653,7 +653,7 @@ static int rebase_config(const char *var, const char *value, void *data)
 		if (git_config_bool(var, value))
 			opts->flags |= REBASE_DIFFSTAT;
 		else
-			opts->flags &= !REBASE_DIFFSTAT;
+			opts->flags &= ~REBASE_DIFFSTAT;
 		return 0;
 	}