utf8: add utf8_strwidth()

I'm about to use this pattern more than once, so make it a common function.

Signed-off-by: Geoffrey Thomas <geofft@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

utf8.c

@@ -246,6 +246,25 @@ int utf8_width(const char **start, size_t *remainder_p)
 	return git_wcwidth(ch);
 }
 
+/*
+ * Returns the total number of columns required by a null-terminated
+ * string, assuming that the string is utf8.  Returns strlen() instead
+ * if the string does not look like a valid utf8 string.
+ */
+int utf8_strwidth(const char *string)
+{
+	int width = 0;
+	const char *orig = string;
+
+	while (1) {
+		if (!string)
+			return strlen(orig);
+		if (!*string)
+			return width;
+		width += utf8_width(&string, NULL);
+	}
+}
+
 int is_utf8(const char *text)
 {
 	while (*text) {

utf8.h

@@ -5,6 +5,7 @@ typedef unsigned int ucs_char_t;  /* assuming 32bit int */
 
 ucs_char_t pick_one_utf8_char(const char **start, size_t *remainder_p);
 int utf8_width(const char **start, size_t *remainder_p);
+int utf8_strwidth(const char *string);
 int is_utf8(const char *text);
 int is_encoding_utf8(const char *name);