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.

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

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho