Added description for reversing git-update-index using --index-info

I wanted to restore a single file from HEAD back to the
index; Shawn Pearce
gave me the answer.  git-ls-tree piped to git-update-index
--index-info.

This patch adds that answer to the git-update-index
documentation.

Signed-off-by: Andy Parkins <andyparkinsgmail.com>
---
 Documentation/git-update-index.txt |   18
++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/Documentation/git-update-index.txt
b/Documentation/git-update-index.txt
index 41bb7e1..5adf717 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
 -215,6
+215,24  for that path.  After the above, we woul
 100755 8a1218a1024a212bb3db30becd860315f9f3ac52 2	frotz
 ------------
 
+One particular use of --index-info is to reverse the effect
of
+"git-update-index frotz":
+
+------------
+git ls-tree HEAD frotz | git update-index --index-info
+------------
+
+This makes the index hold the file frotz from HEAD rather
than from the
+working copy.  Similarly:
+
+------------
+git ls-tree -r HEAD | git update-index --index-info
+------------
+
+Will undo everything except "git add" from the
index, as
+"git-ls-tree -r" lists everything in the last
commit.
+
+
 
 Using "assume unchanged" bit
 ----------------------------
-- 
1.4.3.2

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