lib/commit: Fix min-free-space accounting for duplicate content objects

lib/commit: Fix min-free-space accounting for duplicate content objects

First, in the _ostree_repo_bare_content_commit(), we never actually
checked whether the file was already present in the repo before linking
it in place. Do this so that we can no-op up front.

Second, and the actual bug this patch is fixing: the min-free-space
accounting in write_content_object() and
_ostree_repo_bare_content_commit() optimistically reserves space from
the running `txn.max_blocks` counter _before_ knowing whether we'll
actually no-op or not based on the object already existing.

Over many duplicate writes within a single transaction, the counter
diverges from the actual free space and eventually hits zero, causing a
spurious "min-free-space would be exceeded" error even with plenty of
disk space available.

This may be the source of the CI issue FCOS is hitting in
https://github.com/coreos/fedora-coreos-config/pull/4265 due to the
bootc SELinux relabeling done since
https://github.com/bootc-dev/bootc/pull/2088.

This patch saves the number of blocks reserved and credits them back in
both code paths when a duplicate object is detected.

This was heavily AI-guided, finding the bug, and then doing red-green
testing towards the fix.

Assisted-by: AI