vm-fix-my-summary!!! destroys thread-folding indicators

[Uday Reddy]

When thread-folding is turned on, running vm-fix-my-summary!!! removes the +/- indicators for the folded/unfolded threads.

[Arik (akwm)]

the vm-update-message-summary function was the culprit, there was some complicated
logic which suffered from the 'text-property has already been removed' thing. Since this
function does not first kill the buffer, we can store the thread indicator states prior to re-doing
the summary.

[Uday Reddy (reddyuday)]

Committed to trunk, revision 858.