Initial git18 formula from brew builtin git formula

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
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 `--without-gettext` option.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
subversion: install svnmucc and svnauthz-validate

Install two useful svn tools, svnmucc and svnauthz-validate. User can
disable these tools entirly by passing `--without-tools` option.

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