chars: short-circuit determining the width of characters under U+0300

The combining characters (that are zero-width) start at U+0300.
After that it's pretty much chaos, width-wise.

The mbwidth() function is not called for control characters (whose
representation takes up two columns), as they are handled separately.

The calls of mbwidth() that *can* happen with a control character as
argument are only to determine whether the character is zero-width,
and then it doesn't matter whether the exact width is 1 or 2.

src/chars.c

@@ -179,8 +179,8 @@ char control_mbrep(const char *c, bool isdata)
 /* Return the width in columns of the given (multibyte) character. */
 int mbwidth(const char *c)
 {
-	/* Ask for the width only when the character isn't plain ASCII. */
-	if ((unsigned char)*c > 0xC1) {
+	/* Only characters beyond U+02FF can be other than one column wide. */
+	if ((unsigned char)*c > 0xCB) {
 		wchar_t wc;
 		int width;