gettextize: git-commit "enter the commit message" message

gettextize: git-commit "enter the commit message" message

Gettextize the "# Please enter the commit message for your changes."
message. Several tests in t7500-commit.sh and t7502-commit.sh assume
that this message starts with a newline. Change the tests to to skip
under GETTEXT_POISON=YesPlease.

These fail under GETTEXT_POISON=YesPlease because the poison editor
message doesn't do the right thing with comments and line breaks, so
these messages will be incorrectly broken up across lines.

This test should not be skipped under a hypothetical future testing
mode that tests Git under language locales, since those messages
should all start with a newline like the original.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>