Return Past instead of Present or Future when commit_lsn < min_lsn (#7520)

Implements an approach different from the one #7488 chose: We now return
`past` instead of `present` (or`future`) when encountering the edge case
where commit_lsn < min_lsn. In my opinion, both `past` and `present` are
correct responses, but past is slightly better as the lsn returned by
`present` with #7488 is one too "new". In practice, this shouldn't
matter much, but shrug.

We agreed in slack that this is the better approach:
https://neondb.slack.com/archives/C03F5SM1N02/p1713871064147029

pageserver/src/pgdatadir_mapping.rs

@@ -445,11 +445,6 @@ impl Timeline {
         // include physical changes from later commits that will be marked
         // as aborted, and will need to be vacuumed away.
         let commit_lsn = Lsn((low - 1) * 8);
-        // This maxing operation is for the edge case that the search above did
-        // set found_smaller to true but it never increased the lsn. Then, low
-        // is still the old min_lsn the subtraction above could possibly give a value
-        // below the anchestor_lsn.
-        let commit_lsn = commit_lsn.max(min_lsn);
         match (found_smaller, found_larger) {
             (false, false) => {
                 // This can happen if no commit records have been processed yet, e.g.
@@ -460,6 +455,12 @@ impl Timeline {
                 // Didn't find any commit timestamps smaller than the request
                 Ok(LsnForTimestamp::Past(min_lsn))
             }
+            (true, _) if commit_lsn < min_lsn => {
+                // the search above did set found_smaller to true but it never increased the lsn.
+                // Then, low is still the old min_lsn, and the subtraction above gave a value
+                // below the min_lsn. We should never do that.
+                Ok(LsnForTimestamp::Past(min_lsn))
+            }
             (true, false) => {
                 // Only found commits with timestamps smaller than the request.
                 // It's still a valid case for branch creation, return it.