drop redundant semicolon in empty while

The extra semi-colon is harmless, since we really do want
the while loop to do nothing. But it does trigger a warning
from clang.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

date.c

@@ -586,7 +586,7 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, int *num)
 	const char *end = date;
 	int i;
 
-	while (isalpha(*++end));
+	while (isalpha(*++end))
 		;
 
 	for (i = 0; i < 12; i++) {