Fix staged-files-only for git add --intent-to-add files

pre_commit/git.py

@@ -97,6 +97,20 @@ def get_staged_files():
     )[1])
 
 
+def intent_to_add_files():
+    _, stdout_binary, _ = cmd_output('git', 'status', '--porcelain', '-z')
+    parts = list(reversed(zsplit(stdout_binary)))
+    intent_to_add = []
+    while parts:
+        line = parts.pop()
+        status, filename = line[:3], line[3:]
+        if status[0] in {'C', 'R'}:  # renames / moves have an additional arg
+            parts.pop()
+        if status[1] == 'A':
+            intent_to_add.append(filename)
+    return intent_to_add
+
+
 def get_all_files():
     return zsplit(cmd_output('git', 'ls-files', '-z')[1])