rebase: fix incompatible options error message

rebase: fix incompatible options error message

In commit f57696802c30 ("rebase: really just passthru the `git am`
options", 2018-11-14), the handling of `git am` options was simplified
dramatically (and an option parsing bug was fixed), but it introduced
a small regression in the error message shown when options only
understood by separate backends were used:

$ git rebase --keep --ignore-whitespace
fatal: cannot combine interactive options (--interactive, --exec,
--rebase-merges, --preserve-merges, --keep-empty, --root + --onto) with
am options (.git/rebase-apply/applying)

$ git rebase --merge --ignore-whitespace
fatal: cannot combine merge options (--merge, --strategy,
--strategy-option) with am options (.git/rebase-apply/applying)

Note that in both cases, the list of "am options" is
".git/rebase-apply/applying", which makes no sense.  Since the lists of
backend-specific options is documented pretty thoroughly in the rebase
man page (in the "Incompatible Options" section, with multiple links
throughout the document), and since I expect this list to change over
time, just simplify the error message.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>