DocSettings: Refine the primary/backup mtime shenanigans

Use the same mtime (the newest) for both entries, so that priority order
wins the tie, giving priority to the primary.

Avoids preferring an older legacy sidecar in case of issues with the new
files.

frontend/docsettings.lua

@@ -24,12 +24,18 @@ local function buildCandidates(list)
         -- Ignore missing files.
         if file_path ~= "" and lfs.attributes(file_path, "mode") == "file" then
             local mtime = lfs.attributes(file_path, "modification")
-            -- NOTE: Extra trickery: if we're inserting a "backup" file, and its main buddy exists,
-            --       make sure it will *never* sort ahead of it by negating its mtime.
+            -- NOTE: Extra trickery: if we're inserting a "backup" file, and its primary buddy exists,
+            --       make sure it will *never* sort ahead of it by using the same mtime.
             --       This aims to avoid weird UTC/localtime issues when USBMS is involved,
             --       c.f., https://github.com/koreader/koreader/issues/9227#issuecomment-1345263324
             if file_path:sub(-4) == ".old" and previous_entry_exists then
-                mtime = -mtime
+                local primary_mtime = candidates[#candidates].mtime
+                -- Only proceed with the switcheroo when necessary, and warn about it.
+                if primary_mtime < mtime then
+                    logger.warn("DocSettings: Backup", file_path, "is newer (", mtime, ") than its primary (", primary_mtime, "), fudging timestamps!")
+                    -- Use the most recent timestamp for both (i.e., the backup's).
+                    candidates[#candidates].mtime = mtime
+                end
             end
             table.insert(candidates, {
                     path = file_path,
@@ -44,7 +50,7 @@ local function buildCandidates(list)
     end
 
     -- MRU sort, tie breaker is insertion order (higher priority locations were inserted first).
-    -- Iff a main/backup pair of file both exist, of the two of them, the main one *always* has priority,
+    -- Iff a primary/backup pair of file both exist, of the two of them, the primary one *always* has priority,
     -- regardless of mtime (c.f., NOTE above).
     table.sort(candidates, function(l, r)
                                if l.mtime == r.mtime then