Use a similar file if no old files matches new file name.

Find the old file with the shortest levenshtein distance to the new file
name, and use that for diff operations.

This works great if the file name has version number in it, but even for
a completely new file, using a similar file could still help.

Test: generated a diff
Change-Id: I45062f81772220a5d5872a98a7af01ab69837dcc