Git branch component: Add option to limit the length of the displayed branch name

[Nandax00]

Added a new function that truncates git branch name if the max_length option is set. It also attempts to strip the last incomplete word after the last word separator (i.e. "-", "_").

Demo in tabline with max_length = 30:

• First one's straight-forward.
  branch name = thisisjustonereallylongbranchnamewithoutseparators =>
  
• Here, the 30-long substring would contain "inexplica" at the end, but it is stripped.
  branch name = this-one-contains-an-inexplicably-long-word =>
  

I'm using the following tabline settings:

  tabline = {
    lualine_a = {
      {
        "tabs",
        -- options
        -- ...
      }
    },
    lualine_b = {
      {
        "branch",
        -- NEW OPTION
        max_length = 30,
      },
      "diff",
    },
  },

closes #1289.

[DavidePatria]

you can use fmt option and format a string as you wish

    lualine_b = { {
      'branch',
      fmt = function(str) return format(str, 10) end,
    } }

function format(str, len)
  if string.len(str) > len then
    return string.sub(str,1,len/2) .. '…' .. string.sub(str, str:len()-len/2+1, str:len())
  else
    return str
  end
end

[Nandax00]

Thanks, I definitely missed that part in the docs :D Closing PR and ticket