🐛 truncate_str does not truncate to exact width

[SheldonNico]

• [x ] Please include the raw text output from git, so that we can reproduce the problem.
  (You can use git --no-pager to produce the raw text output.)

diff --git a/bad_width_a.txt b/bad_width_b.txt
index 8b0d958..e69de29 100644
--- a/bad_width_a.txt
+++ b/bad_width_b.txt
@@ -1,5 +0,0 @@
-中文字符长度通常为2
-
-中文字符长度通x常为2
-
-abcdefghijklm j

• [ x] A screenshot of Delta's output is often helpful also.
  

ansi::truncate_str does not truncate str to exact width and cause bad alignment