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>