config_set_store: rename some fields for consistency

config_set_store: rename some fields for consistency

The `seen` field is the actual length of the `offset` array, and the
`offset_alloc` field records what was allocated (to avoid resizing
wherever `seen` has to be incremented).

Elsewhere, we use the convention `name` for the array, where `name` is
descriptive enough to guess its purpose, `name_nr` for the actual length
and `name_alloc` to record the maximum length without needing to resize.

Let's make the names of the fields in question consistent with that
convention.

This will also help with the next steps where we will let the
git_config_set() machinery use the config event stream that we just
introduced.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>