allow user to choose which file to diff if a file does not exist in working tree
tree6b09cb08975c1211f85f481e23b6f4a582e8b2ff
-rw-r--r-- 367 .gitignore
-rw-r--r-- 543 .gitmodules
drwxr-xr-x - Languages
-rw-r--r-- 3425 build.txt
drwxr-xr-x - common
drwxr-xr-x - contrib
drwxr-xr-x - doc
drwxr-xr-x - ext
-rw-r--r-- 145 release.txt
drwxr-xr-x - src
drwxr-xr-x - test