Do not adjust SignColumn highlight

This was introduced in 07d7c9d4 because many colorschemes at the time
had ugly sign columns, and people kept asking how to deal with it.

These days there are many more plugins which use the sign column and
consequently more colorschemes pay attention to the SignColumn highlight
group.

Therefore it no longer makes sense for vim-gitgutter to adjust the
sign column's appearance.

To restore the previous behaviour, add this to your vimrc:

    highlight! link SignColumn LineNr

See #696.

README.mkd

@@ -290,19 +290,14 @@ Please note that vim-gitgutter won't override any colours or highlights you've s
 
 #### Sign column
 
-By default vim-gitgutter will make the sign column look like the line number column.
-
-To customise your sign column's background color, first tell vim-gitgutter to leave it alone:
+Set the `SignColumn` highlight group to change the sign column's colour.  For example:
 
 ```viml
-let g:gitgutter_override_sign_column_highlight = 0
-```
+" vim-gitgutter used to do this by default:
+highlight! link SignColumn LineNr
 
-And then either update your colorscheme's `SignColumn` highlight group or set it in your vimrc:
-
-```viml
-highlight SignColumn ctermbg=whatever    " terminal Vim
-highlight SignColumn guibg=whatever      " gVim/MacVim
+" or you could do this:
+highlight SignColumn guibg=whatever ctermbg=whatever
 ```
 
 By default the sign column will appear when there are signs to show and disappear when there aren't.  To always have the sign column, add to your vimrc:

autoload/gitgutter/highlight.vim

@@ -64,14 +64,6 @@ function! gitgutter#highlight#linenr_toggle() abort
 endfunction
 
 
-function! gitgutter#highlight#define_sign_column_highlight() abort
-  if g:gitgutter_override_sign_column_highlight
-    highlight! link SignColumn LineNr
-  else
-    highlight default link SignColumn LineNr
-  endif
-endfunction
-
 function! gitgutter#highlight#define_highlights() abort
   let [guibg, ctermbg] = s:get_background_colors('SignColumn')
 

doc/gitgutter.txt

@@ -306,7 +306,6 @@ Signs:~
     |g:gitgutter_sign_removed|
     |g:gitgutter_sign_removed_first_line|
     |g:gitgutter_sign_modified_removed|
-    |g:gitgutter_override_sign_column_highlight|
 
 Hunk previews:~
 
@@ -450,20 +449,6 @@ Defaults:
 You can use unicode characters but not images.  Signs must not take up more than
 2 columns.
 
-                                   *g:gitgutter_override_sign_column_highlight*
-Default: 1
-
-Controls whether to make the sign column look like the line-number column (i.e.
-the |hl-LineNr| highlight group).
-
-To customise your sign column's background color, first tell vim-gitgutter to
-leave it alone:
->
-    let g:gitgutter_override_sign_column_highlight = 0
-<
-
-And then either update your colorscheme's |hlSignColumn| highlight group or set
-it in your |vimrc|:
 
   Desired appearance                  Command ~
   Same as line-number column          highlight clear SignColumn
@@ -596,8 +581,11 @@ c. Why can't I unstage staged changes?
 d. Why are the colours in the sign column weird?
 
   Your colorscheme is configuring the |hl-SignColumn| highlight group weirdly.
-  Please see |g:gitgutter_override_sign_column_highlight| on customising the
-  sign column.
+  Here are two ways you could change the colours:
+>
+    highlight! link SignColumn LineNr
+    highlight SignColumn guibg=whatever ctermbg=whatever
+<
 
 e. What happens if I also use another plugin which uses signs (e.g. Syntastic)?
 

plugin/gitgutter.vim

@@ -22,6 +22,13 @@ function! s:set(var, default) abort
   endif
 endfunction
 
+function! s:obsolete(var)
+  if exists(a:var)
+    call gitgutter#utility#warn(a:var.' is obsolete and has no effect.')
+  endif
+endfunction
+
+
 call s:set('g:gitgutter_preview_win_location',     'bo')
 if exists('*nvim_open_win')
   call s:set('g:gitgutter_preview_win_floating', 1)
@@ -44,7 +51,7 @@ if (has('nvim-0.4.0') || exists('*sign_place')) && !exists('g:gitgutter_sign_all
   let g:gitgutter_sign_allow_clobber = 1
 endif
 call s:set('g:gitgutter_sign_allow_clobber',          0)
-call s:set('g:gitgutter_override_sign_column_highlight', 1)
+call s:obsolete('g:gitgutter_override_sign_column_highlight')
 call s:set('g:gitgutter_sign_added',                   '+')
 call s:set('g:gitgutter_sign_modified',                '~')
 call s:set('g:gitgutter_sign_removed',                 '_')
@@ -90,7 +97,6 @@ if !empty(g:gitgutter_grep)
   endif
 endif
 
-call gitgutter#highlight#define_sign_column_highlight()
 call gitgutter#highlight#define_highlights()
 call gitgutter#highlight#define_signs()
 
@@ -267,7 +273,7 @@ augroup gitgutter
     autocmd VimResume * call gitgutter#all(1)
   endif
 
-  autocmd ColorScheme * call gitgutter#highlight#define_sign_column_highlight() | call gitgutter#highlight#define_highlights()
+  autocmd ColorScheme * call gitgutter#highlight#define_highlights()
 
   " Disable during :vimgrep
   autocmd QuickFixCmdPre  *vimgrep* let g:gitgutter_enabled = 0