l10n: git.pot: proposed updates for v1.9 (21+)

The 1st round of Git l10n often starts very late during the Git release
cycle, usually after the release of rc0.  This leads to a giant update
of "po/git.pot" and lots of works for translators with a tight deadline
(a few weeks).  So comes this "pu" branch.

This "pu" branch is based on the "master" branch of Git upstream.  It
will rewind to the "master" branch of upstream and regenerate
"po/git.pot" regularly, so that translators can work on l10n much
earlier.  Because the "pu" branch uses a rewind-and-regenerate strategy,
translators who work on this branch should rebase their work instead of
a fetch/merge.  E.g.:

 1. Create a local tracking branch on "pu", if not have one.

        $ git remote add git-po git://github.com/git-l10n/git-po.git
        $ git fetch git-po
        $ git checkout -b pu git-po/pu

 2. Rebase instead of merge. (Note: In the following example, there is
    only one commit needs to be rebased.)

        $ git fetch git-po
        $ git checkout pu
        $ git rebase -i --onto git-po/pu HEAD~1

 3. Update your "po/XX.po".

        $ msgmerge --add-location --backup=off -U "po/XX.po" \
          po/git.pot

 4. Then start to translate "po/XX.po" and commit.

 5. Do not send pull request to the l10n coordinator for
    translations on this "pu" branch.

 6. When the l10n coordinator starts the 1st round l10n (in the "master"
    branch), translators should cherry-pick translations from "pu"
    to "master" branch.  (Note: In the following example, there is only
    one commit needs to be cherry-picked.)

        $ git checkout master
        $ git pull --ff-only
        $ git cherry-pick pu

 7. Update your "po/XX.po".

        $ msgmerge --add-location --backup=off -U "po/XX.po" \
          po/git.pot

 8. Then start to translate "po/XX.po", commit (--amend), push and
    send pull-request.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>