Only use overbar sign when UI supports it.

Thanks @h3xx.

Closes #471.

autoload/gitgutter/utility.vim

@@ -1,3 +1,11 @@
+function! gitgutter#utility#supports_overscore_sign()
+  if s:windows()
+    return &encoding ==? 'utf-8'
+  else
+    return &termencoding ==? &encoding || &termencoding == ''
+  endif
+endfunction
+
 function! gitgutter#utility#setbufvar(buffer, varname, val)
   let dict = get(getbufvar(a:buffer, ''), 'gitgutter', {})
   let dict[a:varname] = a:val
@@ -201,3 +209,7 @@ function! s:vimdiff(bufnr)
     return 0
   endif
 endfunction
+
+function! s:windows()
+  return has('win64') || has('win32') || has('win16')
+endfunction

plugin/gitgutter.vim

@@ -42,11 +42,12 @@ call s:set('g:gitgutter_override_sign_column_highlight', 1)
 call s:set('g:gitgutter_sign_added',                '+')
 call s:set('g:gitgutter_sign_modified',             '~')
 call s:set('g:gitgutter_sign_removed',              '_')
-try
+
+if gitgutter#utility#supports_overscore_sign()
   call s:set('g:gitgutter_sign_removed_first_line', '‾')
-catch /E239/
-  let g:gitgutter_sign_removed_first_line = '_^'
-endtry
+else
+  call s:set('g:gitgutter_sign_removed_first_line', '_^')
+endif
 
 call s:set('g:gitgutter_sign_modified_removed',    '~_')
 call s:set('g:gitgutter_diff_args',                  '')