Filenames are not tab-completed if "=" present after the option

Important Information

Provide following Information:

• mpv version: 0.32.0+git.20200525T015437.b83bdd1d17
• Linux Distribution and Version: openSUSE Leap 15.2
• Source of the mpv binary: official distro repository
• If known which version of mpv introduced the problem: unknown
• Window Manager and version: plasma5 5.18.5
• GPU driver and version: amdgpu included with distro kernel 5.3.18-lp152.33-default
• Possible screenshot or video of visual glitches: will supply if required

If you're not using git master or the latest release, update.

the relevant file (etc/mpv.bash-completion) appears to be the same as master

Reproduction steps

Try to reproduce your issue with --no-config first. If it isn't reproducible
with --no-config try to first find out which option or script causes your issue.

the problem occurs before launch

Describe the reproduction steps as precise as possible. It's very likely that
the bug you experience wasn't reproduced by the developer because the workflow
differes from your own.

type mpv testfile.mpv --sub-file= and hit tab.

Expected behavior

Filename to be completed.

Actual behavior

Nothing happens.

Log file

Does not apply.

Sample files

Does not apply.

Inspect etc/mpv.bash-completion and reproduce the issue with mpv testfile.mpv --sub-file= followed by Tab. Trace how option values are detected when an equals sign is present, then verify that a filename is completed in this case without breaking existing option completion.