bash completion issue in 16.04

[Geoff Gustafson]

After I upgraded from 14.04 to 16.04 I started seeing this behavior.

I have a makefile that takes a JS argument like so:

make JS=foo/bar.js

If I type make JS=f and then hit Tab to complete, it wipes out my JS= 
and leaves me with this:

make foo/

I can continue completing to the filename but then have to go back and 
add the JS=, which is annoying.

This is happening on 16.04 with bash-completion version 
1:2.1-4.2ubuntu1.1, but doesn't seem to happen on an old 14.04 box with 
1:2.1-4ubuntu0.2.

If I instead tab after just typing JS=, it will give me the full list of 
files/dirs without removing the JS=. If there are no matches to my 
prefix it also leaves my command line alone. But if there are one or 
more matches for the prefix I've typed, it completes that first 
directory or file and wipes out the JS=.

I've tried to debug it, which led me to 
/usr/share/bash-completion/completions/make. But I haven't figured it out.

- Geoff