odb: scan all sources' packfiles before loose objects - #975

The object database refactor that introduced per-source object stores
(cb506a8 "odb: introduce \"files\" source" and the surrounding series,
first released in v2.54.0) changed how do_oid_object_info_extended()
searches for an object. It now iterates the sources and, within each
source, consults that source's packfiles and then its loose object
store before moving on to the next source.

Before that series the search consulted every packfile -- across the
primary object directory and all alternates -- before it looked at any
loose object. The refactor reversed that for the multi-source case:
for an object that lives in an alternate's packfile, the primary
source's loose object store is now consulted first. That loose lookup
is a filesystem stat(), and because callers such as
cache_tree_fully_valid() pass ODB_HAS_OBJECT_RECHECK_PACKED (which
clears OBJECT_INFO_QUICK) the cached-loose-index fast path is skipped
and a real stat() runs for every such object.

In a repository that keeps its objects in an alternate -- the common
arrangement for VFS for Git and Scalar enlistments, where a shared
object cache is mounted as an alternate -- this is a steep penalty.
cache_tree_fully_valid() walks the whole cache tree and calls
odb_has_object() for every node; on an enlistment with a ~2.4M-entry
index that is ~380k objects, each incurring a wasted stat() on the
primary loose store. A same-commit branch switch spent ~32s in
cache_tree_fully_valid() (two calls of ~16s), observed in the field as
a ~2x rise in median checkout duration after the client carrying the
refactor rolled out.

Restore the previous ordering without undoing the per-source
encapsulation: when there is more than one source, scan the packfiles
of every source first (OBJECT_INFO_SKIP_LOOSE) and only then consult
each source's loose store (OBJECT_INFO_SKIP_PACKED). The single-source
case is unchanged, so repositories without alternates keep the
existing path. With the fix the same branch switch spends ~2s in
cache_tree_fully_valid(), the ~380k wasted stat()s are gone, and
performance matches versions predating the refactor.

Assisted-by: Claude Opus 4.8
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>