branch -v: align even when branch names are in UTF-8

Branch names are usually in ASCII so they are not the problem. The
problem most likely comes from "(no branch)" translation, which is
in UTF-8 and makes display-width calculation just wrong.  Clarify
this by renaming the field "len" in struct ref_item to "width", as
it stores the display-width and is used to compute the width of the
screen needed to show the names of all the branches, and compute the
display width using utf8_strwidth(), not byte-length with strlen().

Update document to mention the fact that we may want ref names in
UTF-8. Encodings that produce invalid UTF-8 are safe as utf8_strwidth()
falls back to strlen(). The ones that incidentally produce valid UTF-8
sequences will cause misalignment.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -55,6 +55,8 @@ when you run `git cherry-pick`.
 +
 Note that any of the 'refs/*' cases above may come either from
 the '$GIT_DIR/refs' directory or from the '$GIT_DIR/packed-refs' file.
+While the ref name encoding is unspecified, UTF-8 is prefered as
+some output processing may assume ref names in UTF-8.
 
 '<refname>@\{<date>\}', e.g. 'master@\{yesterday\}', 'HEAD@\{5 minutes ago\}'::
   A ref followed by the suffix '@' with a date specification