Big refactor.

- Hunk stage/undo/preview no longer saves the buffer.
- Hunk undo no longer makes locations go out of sync.
- Grep can be opted out of (grep output with ansi escapes is number one cause
  of issues).
- Replaced g:gitgutter_grep_command with g:gitgutter_grep.
- Always runs git-diff the same way instead of in two possible ways.
- Separated detection of git tracking from diffing.
- Simplified path handling.
- Removed support for xolox shell: Windows taskbar does not flash with async
  jobs.
- Removed g:gitgutter_{eager,realtime}.
- Simplified implementation generally.

autoload/gitgutter/highlight.vim

@@ -1,3 +1,37 @@
+function! gitgutter#highlight#line_disable() abort
+  let g:gitgutter_highlight_lines = 0
+  call s:define_sign_line_highlights()
+
+  if !g:gitgutter_signs
+    call gitgutter#sign#clear_signs(bufnr(''))
+    call gitgutter#sign#remove_dummy_sign(bufnr(''), 0)
+  endif
+
+  redraw!
+endfunction
+
+function! gitgutter#highlight#line_enable() abort
+  let old_highlight_lines = g:gitgutter_highlight_lines
+
+  let g:gitgutter_highlight_lines = 1
+  call s:define_sign_line_highlights()
+
+  if !old_highlight_lines && !g:gitgutter_signs
+    call gitgutter#all(1)
+  endif
+
+  redraw!
+endfunction
+
+function! gitgutter#highlight#line_toggle() abort
+  if g:gitgutter_highlight_lines
+    call gitgutter#highlight#line_disable()
+  else
+    call gitgutter#highlight#line_enable()
+  endif
+endfunction
+
+
 function! gitgutter#highlight#define_sign_column_highlight() abort
   if g:gitgutter_override_sign_column_highlight
     highlight! link SignColumn LineNr
@@ -7,7 +41,7 @@ function! gitgutter#highlight#define_sign_column_highlight() abort
 endfunction
 
 function! gitgutter#highlight#define_highlights() abort
-  let [guibg, ctermbg] = gitgutter#highlight#get_background_colors('SignColumn')
+  let [guibg, ctermbg] = s:get_background_colors('SignColumn')
 
   " Highlights used by the signs.
 
@@ -42,12 +76,12 @@ function! gitgutter#highlight#define_signs() abort
   sign define GitGutterLineModifiedRemoved
   sign define GitGutterDummy
 
-  call gitgutter#highlight#define_sign_text()
+  call s:define_sign_text()
   call gitgutter#highlight#define_sign_text_highlights()
-  call gitgutter#highlight#define_sign_line_highlights()
+  call s:define_sign_line_highlights()
 endfunction
 
-function! gitgutter#highlight#define_sign_text() abort
+function! s:define_sign_text() abort
   execute "sign define GitGutterLineAdded            text=" . g:gitgutter_sign_added
   execute "sign define GitGutterLineModified         text=" . g:gitgutter_sign_modified
   execute "sign define GitGutterLineRemoved          text=" . g:gitgutter_sign_removed
@@ -75,7 +109,7 @@ function! gitgutter#highlight#define_sign_text_highlights() abort
   endif
 endfunction
 
-function! gitgutter#highlight#define_sign_line_highlights() abort
+function! s:define_sign_line_highlights() abort
   if g:gitgutter_highlight_lines
     sign define GitGutterLineAdded            linehl=GitGutterAddLine
     sign define GitGutterLineModified         linehl=GitGutterChangeLine
@@ -91,22 +125,22 @@ function! gitgutter#highlight#define_sign_line_highlights() abort
   endif
 endfunction
 
-function! gitgutter#highlight#get_background_colors(group) abort
+function! s:get_background_colors(group) abort
   redir => highlight
   silent execute 'silent highlight ' . a:group
   redir END
 
   let link_matches = matchlist(highlight, 'links to \(\S\+\)')
   if len(link_matches) > 0 " follow the link
-    return gitgutter#highlight#get_background_colors(link_matches[1])
+    return s:get_background_colors(link_matches[1])
   endif
 
-  let ctermbg = gitgutter#highlight#match_highlight(highlight, 'ctermbg=\([0-9A-Za-z]\+\)')
-  let guibg   = gitgutter#highlight#match_highlight(highlight, 'guibg=\([#0-9A-Za-z]\+\)')
+  let ctermbg = s:match_highlight(highlight, 'ctermbg=\([0-9A-Za-z]\+\)')
+  let guibg   = s:match_highlight(highlight, 'guibg=\([#0-9A-Za-z]\+\)')
   return [guibg, ctermbg]
 endfunction
 
-function! gitgutter#highlight#match_highlight(highlight, pattern) abort
+function! s:match_highlight(highlight, pattern) abort
   let matches = matchlist(a:highlight, a:pattern)
   if len(matches) == 0
     return 'NONE'