i18n: add no-op _() and N_() wrappers

The _ function is for translating strings into the user's chosen
language.  The N_ macro just marks translatable strings for the
xgettext(1) tool without translating them; it is intended for use in
contexts where a function call cannot be used.  So, for example:

	fprintf(stderr, _("Expansion of alias '%s' failed; "
		"'%s' is not a git command\n"),
		cmd, argv[0]);

and

	const char *unpack_plumbing_errors[NB_UNPACK_TREES_ERROR_TYPES] = {
		/* ERROR_WOULD_OVERWRITE */
		N_("Entry '%s' would be overwritten by merge. Cannot merge."),
	[...]

Define such _ and N_ in a new gettext.h and include it in cache.h, so
they can be used everywhere.  Each just returns its argument for now.
_ is a function rather than a macro like N_ to avoid the temptation to
use _("foo") as a string literal (which would be a compile-time error
once _(s) expands to an expression for the translation of s).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -5,6 +5,7 @@
 #include "strbuf.h"
 #include "hash.h"
 #include "advice.h"
+#include "gettext.h"
 
 #include SHA1_HEADER
 #ifndef git_SHA_CTX