backfill: auto-detect sparse-checkout from config

backfill: auto-detect sparse-checkout from config

Commit 85127bcdea ("backfill: assume --sparse when sparse-checkout is
enabled") intended for 'git backfill' to consult the repository
configuration when the user does not pass '--sparse' or
'--no-sparse' on the command line. It added the sentinel check:

    if (ctx->sparse < 0)
        ctx->sparse = cfg->apply_sparse_checkout;

However, the ctx->sparse field is initialized to 0 instead of -1,
so this guard never triggers. Consequently, the repository config
(core.sparseCheckout) is never checked, and the command always
performs a full backfill even when sparse-checkout is enabled.

Fix this by initializing ctx->sparse to -1, ensuring the existing
fallback logic correctly reads the repository configuration when
no explicit flags are provided.

Add a test to verify that 'git backfill' automatically respects
sparse-checkout settings when no flags are passed.

Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>