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 <johannes.schindelin@gmx.de>

Author: dscho

gettext.c

@@ -165,6 +165,9 @@ void git_setup_gettext(void)
 	if (!podir)
 		podir = system_path(GIT_LOCALE_PATH);
 
+	if (!is_directory(podir))
+		return;
+
 	bindtextdomain("git", podir);
 	setlocale(LC_MESSAGES, "");
 	setlocale(LC_TIME, "");