checkout: fix untracked->dir to be reported as UNTRACKED_ADDED

Summary:
When an untracked file was replaced by a directory in the destination commit, eden was reporting a MODIFIED_MODIFIED conflict, but this is not correct. The local file is not tracked, so cannot be modified. This messed up Sapling's expectations.

Step one is to fix eden to return the proper UNTRACKED_ADDED conflict type. Note that Sapling does not currently handle the file->directory path conflict situation here, but I will address that next.

Reviewed By: MichaelCuevas

Differential Revision: D96992296

fbshipit-source-id: 4f73f1612323897bb43c0dd1d0ac132ee275a766

Author: muirdm

eden/fs/service/eden.thrift

@@ -824,27 +824,28 @@ enum CheckoutMode {
 
 enum ConflictType {
   /**
-   * We failed to update this particular path due to an error
+   * We failed to update this particular path due to an error.
    */
   ERROR = 0,
   /**
-   * A locally modified file was deleted in the new Tree
+   * A locally modified file was deleted in the new Tree.
    */
   MODIFIED_REMOVED = 1,
   /**
-   * An untracked local file exists in the new Tree
+   * An untracked local file exists in the new Tree.
+   * The new entry may be a file or directory.
    */
   UNTRACKED_ADDED = 2,
   /**
-   * The file was removed locally, but modified in the new Tree
+   * The file was removed locally, but modified in the new Tree.
    */
   REMOVED_MODIFIED = 3,
   /**
    * The file was removed locally, and also removed in the new Tree.
    */
   MISSING_REMOVED = 4,
   /**
-   * A locally modified file was modified in the new Tree
+   * A locally modified file was modified in the new Tree.
    * This may be contents modifications, or a file type change (directory to
    * file or vice-versa), or permissions changes.
    */