In bff014dac7d9 (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>