i18n: unpack-trees: avoid substituting only a verb in sentences

i18n: unpack-trees: avoid substituting only a verb in sentences

Instead of reusing the same set of message templates for checkout
and other actions and substituting the verb with "%s", prepare
separate message templates for each known action. That would make
it easier for translation into languages where the same verb may
conjugate differently depending on the message we are giving.

See gettext documentation for details:
http://www.gnu.org/software/gettext/manual/html_node/Preparing-Strings.html

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