i18n: rebase-interactive: mark here-doc strings for translation

i18n: rebase-interactive: mark here-doc strings for translation

Use pipe to send gettext output to git stripspace instead of the
original method of using shell here-document, because command
substitution '$(...)' would not take place inside the here-documents.
The exception is the case of the last here-document redirecting to cat,
in which commands substitution works and, thus, is preserved in this
commit.

t3404: adapt test to the strings newly marked for translation
Test t3404-rebase-interactive.sh would fail under GETTEXT_POISON unless
using test_i18ngrep.

Add eval_ngettext fallback functions to be called when running, for
instance, under GETTEXT_POISON. Otherwise, tests would fail under
GETTEXT_POISON, or other build that doesn't support the GNU gettext,
because that function could not be found.

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