[PATCH] fetch: delay fetch_if_missing=0 until after config

When running "git fetch" in a partial clone with no blobs, for example, by:

  git clone --filter=blob:none --no-checkout \
    https://kernel.googlesource.com/pub/scm/git/git
  git -C git fetch

"git fetch" will fail to load the config blob object, printing "unable to load config blob object".

This is because fetch_if_missing is set to 0 before the config is
processed. Git must set fetch_if_missing to 0 before the fetch because
as part of the fetch, packfile negotiation happens (and we do not want
to fetch any missing objects when checking existence of objects), but we
do not need to set it so early. Move the setting of fetch_if_missing to
the earliest possible point in cmd_fetch(), right before any fetching
happens.
---
This is not a full solution, but this helps in the use case described in
the commit message. The full solution probably will involve teaching the
fetch mechanism to support arbitrary struct repository objects, and by
moving fetch_if_missing into the repository object. (Alternatively, we
could add the equivalent of OBJECT_INFO_SKIP_FETCH_OBJECT to functions
like parse_commit() that are used by files like negotiator/default.c, or
split up commit parsing into object reading - which already has that
flag - and commit parsing.)
---
 builtin/fetch.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)