This might be useful to see in general (users will normally only see it after
they quit lazygit again, but still). But it is especially useful when writing
back the config file fails for some reason, because users can then make these
changes manually if they want.
We do this only at startup, when the GUI hasn't started yet. This is probably
good enough, because it is much less likely that writing back a migrated
repo-local config fails because it is not writeable.
It's a bit silly to find out by string comparison whether computeMigratedConfig
did something, when it knows this already and can just return the information.
This doesn't make a huge difference to the production code; the string
comparison isn't very expensive, so this isn't a big deal. However, it makes the
tests clearer; we don't have to bother specifying an expected output string if
the didChange flag is false, and in particular we can get rid of the ugly "This
test intentionally uses non-standard indentation" bit in one of the tests.
