[PATCH] doc: explain default option for rev-parse --short

["André Werlang" <beppe85@gmail.com>]

From 2b1c229153a89c7608e64b87d2f933704c18b7ae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A9=20Werlang?= <beppe85@gmail.com>
Date: Thu, 18 May 2017 10:50:11 -0300
Subject: [PATCH] doc: explain default option for rev-parse --short
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Git 2.11 introduced a computation to guess the default length
for commit short hashes. The documentation isn't updated.

Signed-off-by: André Werlang <beppe85@gmail.com>
---
 Documentation/git-rev-parse.txt | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 7241e96..b49f053 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -139,8 +139,10 @@ can be used.
 --short::
 --short=number::
  Instead of outputting the full SHA-1 values of object names try to
- abbreviate them to a shorter unique name. When no length is specified
- 7 is used. The minimum length is 4.
+ abbreviate them to a shorter unique name. When no length is specified,
+ it is guessed from the number of objects in the repository. In any case,
+ the actual length will be enough to identify the object unambiguously
+ in the current state of the repository. The minimum length is 4.

 --symbolic::
  Usually the object names are output in SHA-1 form (with