Handle file moves for quickfix

Do not assume the new name (`fnamer`) is the same length as the old
name (`fnamel`).

Also use the new name rather than the old name when populating the
quickfix list.

See #872.

autoload/gitgutter.vim

@@ -222,13 +222,11 @@ function! gitgutter#quickfix(current_file)
   let lnum = 0
   for line in diff
     if line =~ '^diff --git [^"]'
-      let paths = line[11:]
-      let mid = (len(paths) - 1) / 2
-      let [fnamel, fnamer] = [paths[:mid-1], paths[mid+1:]]
-      let fname = fnamel ==# fnamer ? fnamel : fnamel[2:]
+      let [fnamel, fnamer] = split(line)[2:3]
+      let fname = fnamel ==# fnamer ? fnamer : fnamer[2:]
     elseif line =~ '^diff --git "'
       let [_, fnamel, _, fnamer] = split(line, '"')
-      let fname = fnamel ==# fnamer ? fnamel : fnamel[2:]
+      let fname = fnamel ==# fnamer ? fnamer : fnamer[2:]
     elseif line =~ '^diff --cc [^"]'
       let fname = line[10:]
     elseif line =~ '^diff --cc "'