ls-tree: add --abbrev[=<n>] option

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-ls-tree.txt

@@ -8,7 +8,9 @@ git-ls-tree - Lists the contents of a tree object
 
 SYNOPSIS
 --------
-'git-ls-tree' [-d] [-r] [-t] [-z] [--name-only] [--name-status] <tree-ish> [paths...]
+'git-ls-tree' [-d] [-r] [-t] [-z]
+	[--name-only] [--name-status] [--full-name] [--abbrev=[<n>]]
+	<tree-ish> [paths...]
 
 DESCRIPTION
 -----------
@@ -40,6 +42,11 @@ OPTIONS
 --name-status::
 	List only filenames (instead of the "long" output), one per line.
 
+--abbrev[=<n>]::
+	Instead of showing the full 40-byte hexadecimal object
+	lines, show only handful hexdigits prefix.
+	Non default number of digits can be specified with --abbrev=<n>.
+
 paths::
 	When paths are given, show them (note that this isn't really raw
 	pathnames, but rather a list of patterns to match).  Otherwise