Fix warning in util.h (signed vs unsigned comparison)

This fix had been delayed by Ramsay because on 32-bit systems it
highlights the fact that off_t is set to an invalid value.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

src/util.h

@@ -38,7 +38,7 @@ extern int git__basename(char *base, size_t n, char *path);
 GIT_INLINE(int) git__is_sizet(off_t p)
 {
 	size_t r = (size_t)p;
-	return p == r;
+	return p == (off_t)r;
 }
 
 /*