Override SignColumn highlight group by default.

Most colorschemes (e.g. solarized) don't give any thought to the
SignColumn highlight group so generally the sign column is ugly.

With this change vim-gitgutter defaults to making the sign column look
like the line number column.

Solarized users no longer need `highlight clear SignColumn` in their
vimrc :)

To stop vim-gitgutter from overriding the SignColumn highlight, add this
to your vimrc:

    let g:gitgutter_override_sign_column_highlight = 0

README.mkd

@@ -183,24 +183,19 @@ Please note that vim-gitgutter won't override any colours or highlights you've s
 
 #### Sign column
 
-The background colour of the sign column is controlled by the `SignColumn` highlight group.  This will be either set in your colorscheme or Vim's default.
+By default vim-gitgutter will make the sign column look like the line number column.
 
-To find out where it's set, and to what it's set, use `:verbose highlight SignColumn`.
+To customise your sign column's background color, first tell vim-gitgutter to leave it alone:
 
-If your `SignColumn` is not set (`:highlight SignColumn` gives you `SignColumn xxx cleared`), vim-gitgutter will set it to the same as your line number column (i.e. the `LineNr` highlight group).
-
-To change your sign column's appearance, update your colorscheme or `~/.vimrc` like this:
-
-* For the same appearance as your line number column: `highlight clear SignColumn`
-* For a specific appearance on terminal Vim: `highlight SignColumn ctermbg=whatever`
-* For a specific appearance on gVim/MacVim: `highlight SignColumn guibg=whatever`
+```viml
+let g:gitgutter_override_sign_column_highlight = 0
+```
 
-If you use [solarized](https://github.com/altercation/vim-colors-solarized)-dark, try the following snippet in your `~/.vimrc`:
+And then either update your colorscheme's `SignColumn` highlight group or set it in your vimrc:
 
 ```viml
-set background=dark
-colorscheme solarized
-highlight clear SignColumn
+highlight SignColumn ctermbg=whatever    " terminal Vim
+highlight SignColumn guibg=whatever      " gVim/MacVim
 ```
 
 By default the sign column will appear when there are signs to show and disappear when there aren't.  If you would always like the sign column to be there, add `let g:gitgutter_sign_column_always = 1` to your `~/.vimrc`.

autoload/gitgutter/highlight.vim

@@ -1,5 +1,9 @@
 function! gitgutter#highlight#define_sign_column_highlight()
-  highlight default link SignColumn LineNr
+  if g:gitgutter_override_sign_column_highlight
+    highlight! link SignColumn LineNr
+  else
+    highlight default link SignColumn LineNr
+  endif
 endfunction
 
 function! gitgutter#highlight#define_highlights()

doc/gitgutter.txt

@@ -130,23 +130,17 @@ set in your colorscheme.
 
 SIGN COLUMN
 
-The background colour of the sign column is controlled by the |hlSignColumn|
-highlight group.  This will be either set in your colorscheme or Vim's default.
+By default vim-gitgutter will make the sign column look like the line number
+column (i.e. the |hl-LineNr| highlight group).
 
-To find out where it's set, and to what it's set, use:
+To customise your sign column's background color, first tell vim-gitgutter to
+leave it alone:
 >
-  :verbose highlight SignColumn
+  let g:gitgutter_override_sign_column_highlight = 0
 <
 
-If your `SignColumn` is not set, i.e if
->
-  :highlight SignColumn  " gives you `SignColumn xxx cleared`
-<
-vim-gitgutter will set it to the same as your line number column (i.e. the
-|hl-LineNr| highlight group).
-
-To change your sign column's appearance, update your colorscheme or |vimrc|
-like this:
+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

plugin/gitgutter.vim

@@ -32,6 +32,7 @@ call s:set('g:gitgutter_max_signs',                 500)
 call s:set('g:gitgutter_signs',                       1)
 call s:set('g:gitgutter_highlight_lines',             0)
 call s:set('g:gitgutter_sign_column_always',          0)
+call s:set('g:gitgutter_override_sign_column_highlight', 1)
 call s:set('g:gitgutter_realtime',                    1)
 call s:set('g:gitgutter_eager',                       1)
 call s:set('g:gitgutter_sign_added',                '+')