Append ".~" to temp file name as file extension when its original file with no extension

This makes it possible to use external diff tool for comparing files with no extension.

(C:\Program Files (x86)\WinMerge\WinMergeU.exe -s -e -x -ul -ur -wl -dl %bname -dr %yname %base %mine)

This may mitigate issue #2924 (closed):

(WinMerge 2011)