Adds a new command spatial-tree index (#475)

* Adds a new command `spatial-tree index`

Command only works if pywraps2 is manually installed into the kart venv.
Although tests exist, they are currently skipped for this reason.
(Not trying to modify the kart build while it is red)

* Address PR comments on spatial-tree command

Author: olsen232

kart/cli.py

@@ -36,6 +36,7 @@
     "pull": {"pull"},
     "resolve": {"resolve"},
     "show": {"create-patch", "show"},
+    "spatial_tree": {"spatial-tree"},
     "status": {"status"},
     "query": {"query"},
     "upgrade": {"upgrade", "upgrade-to-tidy", "upgrade-to-kart"},

kart/repo.py

@@ -47,6 +47,7 @@ class KartRepoFiles:
     # Kart-specific files:
     MERGE_INDEX = "MERGE_INDEX"
     MERGE_BRANCH = "MERGE_BRANCH"
+    S2_INDEX = "s2_index.db"
 
 
 class KartRepoState(Enum):