update-index: list supported idx versions and their features

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index 9d0b1515c58dc825beef6b4507542572c51ca988..dd36d1328554840d2f90739db11ad33b4a677a7e 100644 (file)
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -145,7 +145,15 @@ you will need to handle the situation manually.
 
 --index-version <n>::
        Write the resulting index out in the named on-disk format version.
-       The current default version is 2.
+       Supported versions are 2, 3 and 4. The current default version is 2
+       or 3, depending on whether extra features are used, such as
+       `git add -N`.
++
+Version 4 performs a simple pathname compression that reduces index
+size by 30%-50% on large repositories, which results in faster load
+time. Version 4 is relatively young (first released in in 1.8.0 in
+October 2012). Other Git implementations such as JGit and libgit2
+may not support it yet.
 
 -z::
        Only meaningful with `--stdin` or `--index-info`; paths are