date.c: Fix type missmatch warings from msvc

Fix compiler warings from msvc in date.c for value truncation from 64
bit to 32 bit integers.

Also switch from int to size_t for all variables with result of strlen()
which cannot become negative.

Signed-off-by: Sören Krecker <soekkle@freenet.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -1244,7 +1244,7 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 	}
 
 	for (s = special; s->name; s++) {
-		int len = strlen(s->name);
+		size_t len = strlen(s->name);
 		if (match_string(date, s->name) == len) {
 			s->fn(tm, now, num);
 			*touched = 1;
@@ -1254,7 +1254,7 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 
 	if (!*num) {
 		for (i = 1; i < 11; i++) {
-			int len = strlen(number_name[i]);
+			size_t len = strlen(number_name[i]);
 			if (match_string(date, number_name[i]) == len) {
 				*num = i;
 				*touched = 1;
@@ -1270,7 +1270,7 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
 
 	tl = typelen;
 	while (tl->type) {
-		int len = strlen(tl->type);
+		size_t len = strlen(tl->type);
 		if (match_string(date, tl->type) >= len-1) {
 			update_tm(tm, now, tl->length * *num);
 			*num = 0;