sha1-file: remove OBJECT_INFO_SKIP_CACHED

sha1-file: remove OBJECT_INFO_SKIP_CACHED

In a partial clone, if a user provides the hash of the empty tree ("git
mktree </dev/null" - for SHA-1, this is 4b825d...) to a command which
requires that that object be parsed, for example:

  git diff-tree 4b825d <a non-empty tree>

then Git will lazily fetch the empty tree, unnecessarily, because
parsing of that object invokes repo_has_object_file(), which does not
special-case the empty tree.

Instead, teach repo_has_object_file() to consult find_cached_object()
(which handles the empty tree), thus bringing it in line with the rest
of the object-store-accessing functions. A cost is that
repo_has_object_file() will now need to oideq upon each invocation, but
that is trivial compared to the filesystem lookup or the pack index
search required anyway. (And if find_cached_object() needs to do more
because of previous invocations to pretend_object_file(), all the more
reason to be consistent in whether we present cached objects.)

As a historical note, the function now known as repo_read_object_file()
was taught the empty tree in 346245a1bb ("hard-code the empty tree
object", 2008-02-13), and the function now known as oid_object_info()
was taught the empty tree in c4d9986f5f ("sha1_object_info: examine
cached_object store too", 2011-02-07). repo_has_object_file() was never
updated, perhaps due to oversight. The flag OBJECT_INFO_SKIP_CACHED,
introduced later in dfdd4afcf9 ("sha1_file: teach
sha1_object_info_extended more flags", 2017-06-26) and used in
e83e71c5e1 ("sha1_file: refactor has_sha1_file_with_flags", 2017-06-26),
was introduced to preserve this difference in empty-tree handling, but
now it can be removed.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>