Preserve '[ and '] marks, and &modified, during realtime diff.

See #242.

autoload/gitgutter/diff.vim

@@ -30,7 +30,19 @@ function! gitgutter#diff#run_diff(realtime, use_external_grep)
       let buff_file .= '.'.extension
     endif
     let cmd .= 'git show '.blob_name.' > '.blob_file.' && '
+
+    " Writing the whole buffer resets the '[ and '] marks and also the
+    " 'modified' flag (if &cpoptions includes '+').  These are unwanted
+    " side-effects so we save and restore the values ourselves.
+    let modified      = getbufvar(bufnr, "&mod")
+    let op_mark_start = getpos("'[")
+    let op_mark_end   = getpos("']")
+
     execute 'keepalt silent write' buff_file
+
+    call setbufvar(bufnr, "&mod", modified)
+    call setpos("'[", op_mark_start)
+    call setpos("']", op_mark_end)
   endif
 
   let cmd .= 'git diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args.' -- '

test/keepModified.expected

@@ -0,0 +1,2 @@
+modified: 1
+modified: 1

test/keepOpMarks.expected

@@ -0,0 +1,4 @@
+'[ mark: 0,6,1,0
+'] mark: 0,6,2,0
+'[ mark: 0,6,1,0
+'] mark: 0,6,2,0

test/testKeepModified.vim

@@ -0,0 +1,8 @@
+source helper.vim
+call Setup()
+
+normal 5Go*
+call Dump("modified: ".getbufvar('', '&modified'), 'keepModified')
+doautocmd CursorHold
+call Dump("modified: ".getbufvar('', '&modified'), 'keepModified')
+

test/testKeepOpMarks.vim

@@ -0,0 +1,10 @@
+source helper.vim
+call Setup()
+
+normal 5Go*
+call Dump("'[ mark: ".join(getpos("'["), ','), 'keepOpMarks')
+call Dump("'] mark: ".join(getpos("']"), ','), 'keepOpMarks')
+doautocmd CursorHold
+call Dump("'[ mark: ".join(getpos("'["), ','), 'keepOpMarks')
+call Dump("'] mark: ".join(getpos("']"), ','), 'keepOpMarks')
+