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>