plumbing: use `seen` map in tree walker

This helps avoids iterating down the same trees for every commit.  For
a big-ish repo with 35K objects (17K commits), this reduced the time
for calling `revlist.Objects` during a push (with 0 hashes to ignore)
from more than ten minutes to less than a minute.

1 files changed, 1 insertions, 1 deletions

diff --git a/plumbing/object/file.go b/plumbing/object/file.go
index 79f57fe..40b5206 100644
--- a/plumbing/object/file.go
+++ b/plumbing/object/file.go
@@ -81,7 +81,7 @@ type FileIter struct {
 // NewFileIter takes a storer.EncodedObjectStorer and a Tree and returns a
 // *FileIter that iterates over all files contained in the tree, recursively.
 func NewFileIter(s storer.EncodedObjectStorer, t *Tree) *FileIter {
-	return &FileIter{s: s, w: *NewTreeWalker(t, true)}
+	return &FileIter{s: s, w: *NewTreeWalker(t, true, nil)}
 }
 
 // Next moves the iterator to the next file and returns a pointer to it. If