warn-large-path-threshold: define 0 as number to disable warnings

the default int64_t max was still overflowing for me, when this was dumped as json (noticed during building the manual).
So making 0, the default and define it as "no warnings" fixes the situtation.
Also it's much more human-readable in documentation.

src/libstore/globals.hh

@@ -1227,14 +1227,13 @@ public:
 
     Setting<uint64_t> warnLargePathThreshold{
         this,
-        // n.b. this is deliberately int64 max rather than uint64 max because
-        // this goes through the Nix language JSON parser and thus needs to be
-        // representable in Nix language integers.
-        std::numeric_limits<int64_t>::max(),
+        0,
         "warn-large-path-threshold",
         R"(
           Warn when copying a path larger than this number of bytes to the Nix store
           (as determined by its NAR serialisation).
+          Default is 0, which disables the warning.
+          Set it to 1 to warn on all paths.
         )"
     };
 };

src/libstore/store-api.cc

@@ -171,7 +171,7 @@ std::pair<StorePath, Hash> StoreDirConfig::computeStorePath(
     PathFilter & filter) const
 {
     auto [h, size] = hashPath(path, method.getFileIngestionMethod(), hashAlgo, filter);
-    if (size && *size >= settings.warnLargePathThreshold)
+    if (settings.warnLargePathThreshold && size && *size >= settings.warnLargePathThreshold)
         warn("hashed large path '%s' (%s)", path, renderSize(*size));
     return {
         makeFixedOutputPathFromCA(
@@ -214,7 +214,7 @@ StorePath Store::addToStore(
     auto sink = sourceToSink([&](Source & source) {
         LengthSource lengthSource(source);
         storePath = addToStoreFromDump(lengthSource, name, fsm, method, hashAlgo, references, repair);
-        if (lengthSource.total >= settings.warnLargePathThreshold)
+        if (settings.warnLargePathThreshold && lengthSource.total >= settings.warnLargePathThreshold)
             warn("copied large path '%s' to the store (%s)", path, renderSize(lengthSource.total));
     });
     dumpPath(path, *sink, fsm, filter);