If you tell hg to rename a file, e.g. hg mv foo bar, it will generate a patch which essentially just says “foo was renamed to bar”, and when you look at the diff the only thing that has changed is the name.

If you merge this with a patch that changed foo then hg will do something sensible (ie either merge the changes into bar or give a merge conflict).

Git has no first-class concept of file name changes. Instead it tries to use heuristics to spot renames and sometimes they work and sometimes they won’t. Maybe if you merge a patch renaming foo to bar with one that changes foo the second patch will be applied to bar, but maybe it will behave as if you are merging changes to foo with deletion of foo.

Merging is already hard, dangerous, and non associative. The danger is less that you get lots of annoying merge conflicts than that you don’t get a merge conflict when you should (and therefore you risk accidentally changing the meaning of the merged files without knowing), e.g. if you merge “rename foo to bar” with “delete foo” and git didn’t spot the move then the merge might leave bar untouched when really there should be a conflict between keeping/deleting bar. Having wrong merges happen automatically can be a big risk when software is supposed to be very reliable.