image-tag: Rework the -WIP logic

[dlespiau]

Seems like grepping the output of git status --porcelain broke. Use a
different approach with git diff and only check the events we care about
(having staged and un staged changes).

[leth]

IIRC the reason I changed this was to avoid git reading all the files to generate a diff, which can be slow over a mounted filesystem.

[bboreham]

@leth would you care to explain your regex? ISTM that a regex approach should work, but I had git status --porcelain like this:

 M src/kubernetes.js
 M src/server.js
 M tools/image-tag
?? src/oneweave.js
?? src/oneweave.js~

and it decided that meant there was no WIP.

[leth]

I tested it with that output, and it correctly said -WIP for me, so I'm not sure what is broken.
I think i've overcomplicated the regex though: grep -qE '^[^ ][^ ]\s' would suffice.

[bboreham]

what is happening with this?