subtree: fix split after annotated tag was squashed merged

subtree: fix split after annotated tag was squashed merged

The previous commit fixed a failure in 'git subtree merge --squash' when
the previous squash-merge merged an annotated tag of the subtree
repository which is missing locally.

The same failure happens in 'git subtree split', either directly or when
called by 'git subtree push', under the same circumstances: 'cmd_split'
invokes 'find_existing_splits', which loops through previous commits and
invokes 'git rev-parse' (via 'process_subtree_split_trailer') on the
value of any 'git subtree-split' trailer it finds. This fails if this
value is the hash of an annotated tag which is missing locally.

Add a new optional argument 'repository' to 'cmd_split' and
'find_existing_splits', and invoke 'cmd_split' with that argument from
'cmd_push'. This allows 'process_subtree_split_trailer' to try to fetch
the missing tag from the 'repository' if it's not available locally,
mirroring the new behaviour of 'git subtree pull' and 'git subtree
merge'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>