git: add `without-gettext` option

The localization for Git has been started since v1.7.10. In order to
compile and install Git l10n, must install gettext, and pass the correct
flags to the compiler.

User can disalbe l10n entirly with the `--without-gettext` option.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>