hgmanifest: don't overallocate when loading manifests

Summary: We are using the upperbound of `<[u8]>::split`'s iterator as the value for `with_capacity`, however this is incredibly pessimistic, and as a result we overallocate.  Let's just make an initial pass and count the actual number.

Reviewed By: RajivTS

Differential Revision: D49951367

fbshipit-source-id: c3ce6fa0d4125f2cac26c9ea017a0992f9291397

eden/mononoke/mercurial/types/src/blobs/manifest.rs

@@ -59,11 +59,8 @@ impl ManifestContent {
         data: &[u8],
     ) -> Result<SortedVectorMap<MPathElement, Entry<HgManifestId, (FileType, HgFileNodeId)>>> {
         let lines = data.split(|b| *b == b'\n');
-        let mut files = match lines.size_hint() {
-            // Split returns it count in the high size hint
-            (_, Some(high)) => SortedVectorMap::with_capacity(high),
-            (_, None) => SortedVectorMap::new(),
-        };
+        let count = lines.clone().count();
+        let mut files = SortedVectorMap::with_capacity(count);
 
         for line in lines {
             if line.is_empty() {