Make robust the replacement of temp file paths with actual paths.

The previous implementation meant the temp file paths were treated as
regular expressions, which was vulnerable to problems with backslashes
etc.

See #494.

test/test

@@ -2,6 +2,8 @@
 
 VIM="/Applications/MacVim.app/Contents/MacOS/Vim -v"
 
+export TEST=1
+
 $VIM -u NONE -U NONE -N                      \
   --cmd 'set rtp+=../'                       \
   --cmd 'let g:gitgutter_async=0'            \