Fix independent toggling of signs in sign column and line highlights.

Note that line highlighting requires signs to be placed (because the
line highlight is simply an attribute of a sign).  If the user doesn't
want to see signs in the sign column, but does want line highlighting,
then we make the signs in the sign column invisible.

If neither the signs in the sign column nor the line highlights are
needed (presumably the user just looks at the hunks stats) then we can
remove all signs, at which point Vim removes the sign column...unless
the "sign column always" option is set.

autoload/gitgutter.vim

@@ -23,7 +23,7 @@ function! gitgutter#process_buffer(file, realtime)
         call hunk#set_hunks(diff#parse_diff(diff))
         let modified_lines = diff#process_hunks(hunk#hunks())
 
-        if g:gitgutter_signs
+        if g:gitgutter_signs || g:gitgutter_highlight_lines
           call sign#update_signs(a:file, modified_lines)
         endif
 
@@ -77,19 +77,34 @@ endfunction
 function! gitgutter#line_highlights_disable()
   let g:gitgutter_highlight_lines = 0
   call highlight#define_sign_line_highlights()
+
+  if !g:gitgutter_signs
+    call sign#clear_signs(utility#file())
+    call sign#remove_dummy_sign(0)
+  endif
+
   redraw!
 endfunction
 
 function! gitgutter#line_highlights_enable()
+  let old_highlight_lines = g:gitgutter_highlight_lines
+
   let g:gitgutter_highlight_lines = 1
   call highlight#define_sign_line_highlights()
+
+  if !old_highlight_lines && !g:gitgutter_signs
+    call gitgutter#all()
+  endif
+
   redraw!
 endfunction
 
 function! gitgutter#line_highlights_toggle()
-  let g:gitgutter_highlight_lines = !g:gitgutter_highlight_lines
-  call highlight#define_sign_line_highlights()
-  redraw!
+  if g:gitgutter_highlight_lines
+    call gitgutter#line_highlights_disable()
+  else
+    call gitgutter#line_highlights_enable()
+  endif
 endfunction
 
 " }}}
@@ -97,14 +112,24 @@ endfunction
 " Signs {{{
 
 function! gitgutter#signs_enable()
+  let old_signs = g:gitgutter_signs
+
   let g:gitgutter_signs = 1
-  call gitgutter#all()
+  call highlight#define_sign_text_highlights()
+
+  if !old_signs && !g:gitgutter_highlight_lines
+    call gitgutter#all()
+  endif
 endfunction
 
 function! gitgutter#signs_disable()
   let g:gitgutter_signs = 0
-  call sign#clear_signs(utility#file())
-  call sign#remove_dummy_sign(0)
+  call highlight#define_sign_text_highlights()
+
+  if !g:gitgutter_highlight_lines
+    call sign#clear_signs(utility#file())
+    call sign#remove_dummy_sign(0)
+  endif
 endfunction
 
 function! gitgutter#signs_toggle()

autoload/highlight.vim

@@ -6,17 +6,24 @@ function! highlight#define_highlights()
   let [guibg, ctermbg] = highlight#get_background_colors('SignColumn')
 
   " Highlights used by the signs.
+
   execute "highlight GitGutterAddDefault    guifg=#009900 guibg=" . guibg . " ctermfg=2 ctermbg=" . ctermbg
   execute "highlight GitGutterChangeDefault guifg=#bbbb00 guibg=" . guibg . " ctermfg=3 ctermbg=" . ctermbg
   execute "highlight GitGutterDeleteDefault guifg=#ff2222 guibg=" . guibg . " ctermfg=1 ctermbg=" . ctermbg
   highlight default link GitGutterChangeDeleteDefault GitGutterChangeDefault
 
+  execute "highlight GitGutterAddInvisible    guifg=bg" . " guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
+  execute "highlight GitGutterChangeInvisible guifg=bg" . " guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
+  execute "highlight GitGutterDeleteInvisible guifg=bg" . " guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
+  highlight default link GitGutterChangeDeleteInvisible GitGutterChangeInvisble
+
   highlight default link GitGutterAdd          GitGutterAddDefault
   highlight default link GitGutterChange       GitGutterChangeDefault
   highlight default link GitGutterDelete       GitGutterDeleteDefault
   highlight default link GitGutterChangeDelete GitGutterChangeDeleteDefault
 
   " Highlights used for the whole line.
+
   highlight default link GitGutterAddLine          DiffAdd
   highlight default link GitGutterChangeLine       DiffChange
   highlight default link GitGutterDeleteLine       DiffDelete
@@ -43,6 +50,21 @@ function! highlight#define_sign_text()
 endfunction
 
 function! highlight#define_sign_text_highlights()
+  " Once a sign's text attribute has been defined, it cannot be undefined or
+  " set to an empty value.  So to make signs' text disappear we make it
+  " invisible.
+  if g:gitgutter_signs
+    highlight link GitGutterAdd          GitGutterAddDefault
+    highlight link GitGutterChange       GitGutterChangeDefault
+    highlight link GitGutterDelete       GitGutterDeleteDefault
+    highlight link GitGutterChangeDelete GitGutterChangeDeleteDefault
+  else
+    highlight link GitGutterAdd          GitGutterAddInvisible
+    highlight link GitGutterChange       GitGutterChangeInvisible
+    highlight link GitGutterDelete       GitGutterDeleteInvisible
+    highlight link GitGutterChangeDelete GitGutterChangeDeleteInvisible
+  endif
+
   sign define GitGutterLineAdded           texthl=GitGutterAdd
   sign define GitGutterLineModified        texthl=GitGutterChange
   sign define GitGutterLineRemoved         texthl=GitGutterDelete