update: better handle merge conflicts.
update: better handle merge conflicts.

When there are merge conflicts we fail pretty hard. This is still
possible after this commit but at least we've given Git enough pointers
to make it less likely.

Closes #49299.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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