filter-branch: fix behaviour of '-k'
The option '-k' says that the given commit and _all_ of its ancestors
are kept as-is.
However, if a to-be-rewritten commit branched from an ancestor of an
ancestor of a commit given with '-k', filter-branch would fail.
Example:
A - B
\
C
If filter-branch was called with '-k B -s C', it would actually keep
B (and A as its parent), but would rewrite C, and its parent.
Noticed by Johannes Sixt.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>