fetch: do not override partial clone filter

fetch: do not override partial clone filter

When a fetch with the --filter argument is made, the configured default
filter is set even if one already exists. This change was made in
5e46139376 ("builtin/fetch: remove unique promisor remote limitation",
2019-06-25) - in particular, changing from:

 * If this is the FIRST partial-fetch request, we enable partial
 * on this repo and remember the given filter-spec as the default
 * for subsequent fetches to this remote.

to:

 * If this is a partial-fetch request, we enable partial on
 * this repo if not already enabled and remember the given
 * filter-spec as the default for subsequent fetches to this
 * remote.

(The given filter-spec is "remembered" even if there is already an
existing one.)

This is problematic whenever a lazy fetch is made, because lazy fetches
are made using "git fetch --filter=blob:none", but this will also happen
if the user invokes "git fetch --filter=<filter>" manually. Therefore,
restore the behavior prior to 5e46139376, which writes a filter-spec
only if the current fetch request is the first partial-fetch one (for
that remote).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>