i18n: rebase-interactive: mark comments of squash for translation

i18n: rebase-interactive: mark comments of squash for translation

Mark comment messages of squash/fixup file ($squash_msg) for
translation.

Helper functions this_nth_commit_message and skip_nth_commit_message
replace the previous method of making the comment messages (such as
"This is the 2nd commit message:") aided by nth_string helper function.
This step was taken as a workaround to enabled translation of entire
sentences. However, doesn't change any text seen in English by the user,
except for string "The first commit's message is:" which was changed to
match the style of other instances.

The test t3404-rebase-interactive.sh resorts to set_fake_editor which
didn't account for GETTEXT_POISON. Fix it by assuming success when we
find dummy gettext poison output where was supposed to find the first
comment line "This is a combination of $count commits.".

For that same message, use plural aware eval_ngettext instead of
eval_gettext, since other languages have more complex plural forms.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>