refs/files: remove "name exist?" check in lock_ref_oid_basic()

In lock_ref_oid_basic() we'll happily lock a reference that doesn't exist yet. That's normal, and is how references are initially born, but we don't need to retain checks here in lock_ref_oid_basic() about the state of the ref, when what we're checking is either checked already, or something we're about to discover by trying to lock the ref with raceproof_create_file(). The one exception is the caller in files_reflog_expire(), who passes us a "type" to find out if the reference is a symref or not. We can move the that logic over to that caller, which can now defer its discovery of whether or not the ref is a symref until it's needed. In the preceding commit an exhaustive regression test was added for that case in a new test in "t1417-reflog-updateref.sh". The improved diagnostics here were added in 5b2d8d6f218 (lock_ref_sha1_basic(): improve diagnostics for ref D/F conflicts, 2015-05-11), and then much of the surrounding code went away recently in my 245fbba46d6 (refs/files: remove unused "errno == EISDIR" code, 2021-08-23). The refs_resolve_ref_unsafe() code being removed here looks like it should be tasked with doing that, but it's actually redundant to other code. The reason for that is as noted in 245fbba46d6 this once widely used function now only has a handful of callers left, which all handle this case themselves. To the extent that we're racy between their check and ours removing this check actually improves the situation, as we'll be doing fewer things between the not-under-lock initial check and acquiring the lock. Why this is OK for all the remaining callers of lock_ref_oid_basic() is noted below. There are only two of those callers: * "git branch -[cm] <oldbranch> <newbranch>": In files_copy_or_rename_ref() we'll call this when we copy or rename refs via rename_ref() and copy_ref(). but only after we've checked if the refname exists already via its own call to refs_resolve_ref_unsafe() and refs_rename_ref_available(). As the updated comment to the latter here notes neither of those are actually needed. If we delete not only this code but also refs_rename_ref_available() we'll do just fine, we'll just emit a less friendly error message if e.g. "git branch -m A B/C" would have a D/F conflict with a "B" file. Actually we'd probably die before that in case reflogs for the branch existed, i.e. when the try to rename() or copy_file() the relevant reflog, since if we've got a D/F conflict with a branch name we'll probably also have the same with its reflogs (but not necessarily, we might have reflogs, but it might not). As some #leftoverbits that code seems buggy to me, i.e. the reflog "protocol" should be to get a lock on the main ref, and then perform ref and/or reflog operations. That code dates back to c976d415e53 (git-branch: add options and tests for branch renaming, 2006-11-28) and probably pre-dated the solidifying of that convention. But in any case, that edge case is not our bug or problem right now. * "git reflog expire <ref>": In files_reflog_expire() we'll call this without previous ref existence checking in files-backend.c, but that code is in turn called by code that's just finished checking if the refname whose reflog we're expiring exists. See ae35e16cd43 (reflog expire: don't lock reflogs using previously seen OID, 2021-08-23) for the current state of that code, and 5e6f003ca8a (reflog_expire(): ignore --updateref for symbolic references, 2015-03-03) for the code we'd break if we only did a "update = !!ref" here, which is covered by the aforementioned regression test in "t1417-reflog-updateref.sh". Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>