[bash-autocompletion] Fix bug when a flag ends with '='
There was a bug that when a flag ends with '=' and no value was suggested,
clang autocompletes the flag itself.
For example, in bash, it looked like this:
$ clang -fmodule-file=[tab] -> $clang -fmodule-file=-fmodule-file
This is not what we expect. We expect a file autocompletion when no value
was found. With this patch, pressing tab suggests files in the current
Reviewers: teemperor, ruiu