gettext: handle GIT_TEXTDOMAINDIR relative to $(prefix)

On Windows, there is no single root directory. And what Git thinks is a
root directory is not a root directory at all: everything is relative to
the location where Git is installed.

To handle this situation better, let's just allow for GIT_TEXTDOMAINDIR
to be a path relative to the (runtime) prefix.

To that end, we have to switch the order in which common-main handles
argv0 and sets up gettext: in order to have access to the runtime
prefix, we need it to be inferred from argv0 already.

This patch also prepares for GIT_LOCALE_PATH to be relative to prefix,
which is the even more important fix.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

common-main.c

@@ -32,12 +32,12 @@ int main(int argc, const char **argv)
 	 */
 	sanitize_stdfds();
 
+	git_extract_argv0_path(argv[0]);
+
 	git_setup_gettext();
 
 	attr_start();
 
-	git_extract_argv0_path(argv[0]);
-
 	restore_sigpipe_to_default();
 
 	return cmd_main(argc, argv);

gettext.c

@@ -6,6 +6,8 @@
 #include "gettext.h"
 #include "strbuf.h"
 #include "utf8.h"
+#include "cache.h"
+#include "exec_cmd.h"
 
 #ifndef NO_GETTEXT
 #	include <locale.h>
@@ -158,14 +160,20 @@ static void init_gettext_charset(const char *domain)
 void git_setup_gettext(void)
 {
 	const char *podir = getenv("GIT_TEXTDOMAINDIR");
+	char *p = NULL;
 
 	if (!podir)
 		podir = GIT_LOCALE_PATH;
+	if (!is_absolute_path(podir))
+		podir = p = system_path(podir);
+
 	bindtextdomain("git", podir);
 	setlocale(LC_MESSAGES, "");
 	setlocale(LC_TIME, "");
 	init_gettext_charset("git");
 	textdomain("git");
+
+	free(p);
 }
 
 /* return the number of columns of string 's' in current locale */