git: add optional gettext dep

The localization for Git has been started since v1.7.10, and it needs
gettext to work properly. To install git with l10n translations, run
this command:

    brew install --with-gettext git

It will install the keg-only gettext dep, set the proper CPPFLAGS and
LDFLAGS ENV automatically.

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