gettext: add gettextln, eval_gettextln to encode common idiom

gettext: add gettextln, eval_gettextln to encode common idiom

Currently, if you want to use gettext or eval_gettext to format a message
you may have to add a separate echo statement and a surrounding subshell
in order to interpolate the required trailing new line.

This patch introduces two new helper functions, gettextln and eval_gettextln
which append a trailing newline to the gettext output.

This allows constructions of the form:

	if test -s "$GIT_DIR/BISECT_START"
	then
		(
			gettext "You need to give me at least one good and one bad revisions.
(You can use \"git bisect bad\" and \"git bisect good\" for that.)" &&
			echo
		) >&2
	else
	...

to be expressed more concisely as:

	if test -s "$GIT_DIR/BISECT_START"
	then
		gettextln "You need to give me at least one good and one bad revisions.
(You can use \"git bisect bad\" and \"git bisect good\" for that.)" >&2
	else
	...

Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>