Before this, += and -= worked well when applied to keys defined in the
current section or in a section of the same name inherited from a file
extended by the current file, but not to keys inherited from another
section using macro extension <=, because <= substitution happens later
and all the += and -= are already resolved.

e.g.:

```
[macro]
a = 1

[part]
<= macro
a += 2
```

was equivalent to

```
[part]
a = 1
a = 2
```

instead of

```
[part]
a = 1
a += 2
```

Now a partial and brittle support for this is enabled by postponing
+= and -= resolution until <= substitution happens when the current
section contains a <= extension. But this is not guaranteed: if the
current section is overloaded in another file extending the current
one, then += and -= resolution will happend then, before <=.

A consistent solution would be to unify the implementation of
extends= and <= and update sections by taking both into account,
instead of updating based only on extends and computing <= later,
leading to inconsistencies. This could be achieved e.g. by computing
section updates on demand during substitution.
Co-authored-by: Xavier Thompson <xavier.thompson@nexedi.com>