Makefile: add "po-init" rule to initialize po/XX.po

Makefile: add "po-init" rule to initialize po/XX.po

The core translation is the minimum set of work that must be done for a
new language translation.

There are over 5000 messages in the template message file "po/git.pot"
that need to be translated. It is not a piece of cake for such a huge
workload. So we used to define a small set of messages called "core
translation" that a new l10n contributor must complete before sending
pull request to the l10n coordinator.

By pulling in some parts of the git-po-helper[^1] logic, we add a new
rule to create this core translation message "po/git-core.pot":

    make po/git-core.pot

To help new l10n contributors to initialized their "po/XX.pot" from
"po/git-core.pot", we also add new rules "po-init":

    make po-init PO_FILE=po/XX.po

[^1]: https://github.com/git-l10n/git-po-helper/

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