gettext: avoid initialization if the locale dir is not present

gettext: avoid initialization if the locale dir is not present

The runtime of a simple `git.exe version` call on Windows is currently
dominated by the gettext setup, adding a whopping ~150ms to the ~210ms
total.

Given that this cost is added to each and every git.exe invocation goes
through common-main's invocation of git_setup_gettext(), and given that
scripts have to call git.exe dozens, if not hundreds, of times, this is
a substantial performance penalty.

This is particularly pointless when considering that Git for Windows
ships without localization (to keep the installer's size to a bearable
~34MB): all that time setting up gettext is for naught.

To be clear, Git for Windows *needs* to be compiled with localization,
for the following reasons:

- to allow users to copy add-on localization in case they want it, and

- to fix the nasty error message

	BUG: your vsnprintf is broken (returned -1)

  by using libgettext's override of vsnprintf() that does not share the
  behavior of msvcrt.dll's version of vsnprintf().

So let's be smart about it and skip setting up gettext if the locale
directory is not even present.

Since localization might be missing for not-yet-supported locales, this
will not break anything.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>