dispatch: fix silly blackbox entries when hg is interrupted
When hg is interrupted, it creates ui.log like this:
1970/01/01 00:00:00 user @
0000000000000000000000000000000000000000 (62488)> killed!
exited 255 after 1.78 seconds
This is due to a scoping problem: two different uses of the name "msg"
collide. So rename one of them.
Differential Revision: https://phab.mercurial-scm.org/D12427
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