Temporarily disable tree entry pool on Windows

Recent tests and benchmarks have shown that this can have an dramatic
adverse effect on memory consumption and fragmentation in high
concurrency scenarios. malloc on Windows already has built in support
for managing multiple small allocations and the Low-fragmentation Heap
is possibly helping here as well but being subverted by the pool.
diff --git a/src/tree.c b/src/tree.c
index 48b9f12..5342cbc 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -466,7 +466,11 @@
 		filename_len = nul - buffer;
 		/** Allocate the entry and store it in the entries vector */
 		{
+#ifdef GIT_WIN32
+			entry = alloc_entry(buffer);
+#else
 			entry = alloc_entry_pooled(&tree->pool, buffer, filename_len);
+#endif
 			GITERR_CHECK_ALLOC(entry);
 
 			if (git_vector_insert(&tree->entries, entry) < 0)