Merge branch 'gc/index-format-doc'

Doc update.

* gc/index-format-doc:
  docs: document zero bits in index "mode"

Author: gitster

Documentation/gitformat-index.txt

@@ -83,11 +83,13 @@ Git index format
 
   32-bit mode, split into (high to low bits)
 
+    16-bit unused, must be zero
+
     4-bit object type
       valid values in binary are 1000 (regular file), 1010 (symbolic link)
       and 1110 (gitlink)
 
-    3-bit unused
+    3-bit unused, must be zero
 
     9-bit unix permission. Only 0755 and 0644 are valid for regular files.
     Symbolic links and gitlinks have value 0 in this field.