update: clarify calculateupdate() call sites by specifying argument names
merge.calculateupdate() takes a lot of parameters and I get confused
all the time which is which.
See also
b14fdf1fb615 (update: clarify update() call sites by
specifying argument names, 2017-02-09).
Differential Revision: https://phab.mercurial-scm.org/D6883
local test passed
addgroupcopy test passed
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lowlevelcopy test passed
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