simplemerge: make minimize() a free function
IMO, `Merge3Text` should be about merging text without knowing about
conflict markers. "Conflict minimization" is about making conflict
markers smaller, so it should be moved out. This patch does that. I'll
refactor it next.
Differential Revision: https://phab.mercurial-scm.org/D11980
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed