We'd like to support localization in Git LFS.  To do so, add the gotext
package, which implement gettext-compatible localization.

We'd like to support localizing Git LFS.  We have the gotext package,
which supports several different ways of getting GNU gettext-compatible
locale strings.  However, we need some central message catalog we can
use.

Let's create a singleton by the name of Tr.  That means that we can
translate a string by writing something like this:

  fmt.Println(tr.Tr.Get("This is a message about %s.", subject))

These strings can be automatically extracted by the xgotext tool
available from the package
github.com/leonelquinteros/gotext/cli/xgotext.  While currently quite
slow, it does understand Go syntax, unlike the standard GNU gettext
suite.

We initialize it based on the locale.  Since Go does not use the
standard C library, we have to emulate its behavior, as outlined in
locale(7).  If our locale is "en_DK.UTF-8", we first look for "en_DK"
and then "en", in that order.  This preserves a fallback for languages
which are similar across most speakers, like English and Spanish, and
can share a translation, but for locales which cannot, such pt_PT and
pt_BR, or zh_TW and zh_CN, this ensures we prefer a more appropriate
locale.

Introduce a variable called "locales" which will be used to store
Base64-encoded MO files (compiled Gettext files) for each locale.  We
can still load external locales from the standard directory
(/usr/share/locale), but this preserves the behavior of providing a
single, relocatable binary, which people like.  If this becomes a
problem in the future, we can revisit it.  Population of this variable
will come in a future commit.

As the first thing we do when we run a command, let's initialize the
locale.  This will let us print any sort of error or warning messages in
the user's preferred language if we have any problems.

It will take some time for us to get the codebase translated.  However,
as a test of our codepaths and as a demonstration of what to do, let's
introduce a single translation, plus a simple Spanish translation that
translates only that string.  This allows an invocation like the
following to print a simple translated message:

  LC_ALL=es_ES.UTF-8 git lfs filter-process

When building the default image in CI, or a release build, ensure that
the gettext package is installed so we can test locales.  We don't
include it in all CI runs to ensure that we also test building without
localization.

Note that on Windows, these tools should always be installed as part of
the Git for Windows environment, since they are needed for Git, and we
would have no way to install them anyway.