This is consistent with `git commit` which, like `git merge`, supports
passing the commit message via `-m <msg>` and, unlike `git merge` before
this patch, via `-F <file>`.
It is useful to allow this for scripted use, or for the upcoming patch
to allow (re-)creating octopus merges in `git rebase --rebase-merges`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>