basic/escape: allow truncation mode where "…" is always appended

So far we would append "…" or "..." when the string was wider than the specified
output width. But let's add a mode where the caller knows that the string being
passed is already truncated.

The condition for jumping back in utf8_escape_non_printable_full() was
off-by-one. But we only jumped to that label after doing a check with a
stronger condition, so I think it didn't matter. Now it matters because we'd
output the forced ellipsis one column too early.

src/basic/utf8.c

@@ -212,7 +212,7 @@ static int utf8_char_console_width(const char *str) {
         return unichar_iswide(c) ? 2 : 1;
 }
 
-char *utf8_escape_non_printable_full(const char *str, size_t console_width) {
+char *utf8_escape_non_printable_full(const char *str, size_t console_width, bool force_ellipsis) {
         char *p, *s, *prev_s;
         size_t n = 0; /* estimated print width */
 
@@ -229,8 +229,12 @@ char *utf8_escape_non_printable_full(const char *str, size_t console_width) {
                 int len;
                 char *saved_s = s;
 
-                if (!*str) /* done! */
-                        goto finish;
+                if (!*str) { /* done! */
+                        if (force_ellipsis)
+                                goto truncation;
+                        else
+                                goto finish;
+                }
 
                 len = utf8_encoded_valid_unichar(str, SIZE_MAX);
                 if (len > 0) {
@@ -274,7 +278,7 @@ char *utf8_escape_non_printable_full(const char *str, size_t console_width) {
 
  truncation:
         /* Try to go back one if we don't have enough space for the ellipsis */
-        if (n + 1 >= console_width)
+        if (n + 1 > console_width)
                 s = prev_s;
 
         s = mempcpy(s, "…", strlen("…"));