Merge proposal comment should be restored after 'Claim' is clicked

[Jamu Kakar]

Sometimes I forget to click 'Claim' on a merge proposal before I
start writing review comments about the branch. If the review will
take some time I like to click 'Claim' before I'm done to make sure
other people that might want to review the branch know what I'm
doing. It'd be nice if text in the comment box was restored (but
not posted as a comment) after I click 'Claim', so that I would (a)
not lose and data and (b) could click 'Claim' at any time knowning
that I could continue where I left off.

Of course, a better solution than this would be to make the clicking
of 'Claim' use AJAX to seamlessly update the page, so perhaps that's
the real problem that ought to be fixed.