Add localization support

[DJm00n]

We need to support l10n for Git.
Upstream Git uses gettext po files for localization of console git, git-gui and gitk.
MSYS git uses them too:

[nalla]

This is a duplicate of #378.

Nevertheless, you can use the SDK to build yourself an installer that will have localization support.

WITH_I18N=1 /usr/src/build-extra/installer/release.sh 2.6.1-with-i18n.

[dscho]

@DJm00n It would have been nice if you had worked a little bit toward fulfilling your wish.

For starters, this is a duplicate of #403 and of #378.

Also, I had just added this commit: git-for-windows/build-extra@f54833d

And finally, you could have investigated a little bit why we exclude the language support. It was done to save space: #262

As it happens, I was curious and built an installer with, and another without, localized messages. The difference is 4MB, nothing to laugh about when the installer is a mere 29MB to begin with.

[dscho]

Oh yeah, and the plan is to automate the release engineering quite a bit. When that finally is said and done, I will add installers with multi-language support.

Feel free to help make that happen by working on https://github.com/git-for-windows/build-extra/compare/cmd-scripts