fix(blame): avoid right-aligned blame overlapping buftext

When the blame text's length exceeds the available space to
the right of the buffer's text, the intention is to switch
to the 'eol' extmark placement. However, there were a number
of issues that could trip up the time at which it swtiches
to 'eol':

- if the buffer line or virtual text contain multibyte characters,
  they weren't counted properly in terms of screen cells that they'd
  consume
- incorrect window identifer was passed when calculating the
  available space, meaning that signs/folds/numbers columns
  weren't properly accounted for

lua/gitsigns/current_line_blame.lua

@@ -43,8 +43,8 @@ end
 
 --- @param winid integer
 --- @return integer
-local function win_width(winid)
-  winid = winid or api.nvim_get_current_win()
+local function win_width()
+  local winid = api.nvim_get_current_win()
   local wininfo = vim.fn.getwininfo(winid)[1]
   local textoff = wininfo and wininfo.textoff or 0
   return api.nvim_win_get_width(winid) - textoff
@@ -54,7 +54,8 @@ end
 --- @param lnum integer
 --- @return integer
 local function line_len(bufnr, lnum)
-  return #api.nvim_buf_get_lines(bufnr, lnum - 1, lnum, true)[1]
+  local line = api.nvim_buf_get_lines(bufnr, lnum - 1, lnum, true)[1]
+  return api.nvim_strwidth(line)
 end
 
 --- @param fmt string
@@ -103,7 +104,7 @@ local function handle_blame_info(bufnr, lnum, blame_info, opts)
   if opts.virt_text then
     local virt_text_pos = opts.virt_text_pos
     if virt_text_pos == 'right_align' then
-      if #virt_text_str > (win_width(0) - line_len(bufnr, lnum)) then
+      if api.nvim_strwidth(virt_text_str) > (win_width() - line_len(bufnr, lnum)) then
         virt_text_pos = 'eol'
       end
     end