Consider this git config --list case:
url.git+ssh://git@example.com/.insteadOf=gl
url.git+ssh://git@example.com/.insteadOf=shared
Since config is stored in a Map, only the last of the values for this key
was stored and available for use by the insteadOf code. But that
is wrong; git allows either "gl" or "shared" to be used in an url and
the insteadOf value to be substituted in.
To support this, it seems best to keep the existing config map as-is,
and add a second map that accumulates a list of multiple values for
config keys. This new fullconfig map can be used in the rare places where
multiple values for a key make sense, without needing to complicate
everything else.
Haskell's laziness and data sharing keep the overhead of adding
this second map low.
