fetch: allow adding a filter after initial clone

Retroactively adding a filter can be useful for existing shallow clones as
they allow users to see earlier change histories without downloading all
git objects in a regular --unshallow fetch.

Without this patch, users can make a clone partial by editing the
repository configuration to convert the remote into a promisor, like:

  git config core.repositoryFormatVersion 1
  git config extensions.partialClone origin
  git fetch --unshallow --filter=blob:none origin

Since the hard part of making this work is already in place and such
edits can be error-prone, teach Git to perform the required configuration
change automatically instead.

Note that this change does not modify the existing git behavior which
recognizes setting extensions.partialClone without changing
repositoryFormatVersion.

Signed-off-by: Xin Li <delphij@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5702-protocol-v2.sh

@@ -348,7 +348,6 @@ test_expect_success 'partial fetch' '
 	rm -rf client "$(pwd)/trace" &&
 	git init client &&
 	SERVER="file://$(pwd)/server" &&
-	test_config -C client extensions.partialClone "$SERVER" &&
 
 	GIT_TRACE_PACKET="$(pwd)/trace" git -C client -c protocol.version=2 \
 		fetch --filter=blob:none "$SERVER" master:refs/heads/other &&