blame: document --contents option

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-blame.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git-blame' [-c] [-l] [-t] [-f] [-n] [-p] [--incremental] [-L n,m] [-S <revs-file>]
-            [-M] [-C] [-C] [--since=<date>] [<rev>] [--] <file>
+            [-M] [-C] [-C] [--since=<date>] [<rev> | --contents <file>] [--] <file>
 
 DESCRIPTION
 -----------
@@ -67,6 +67,13 @@ OPTIONS
 	Show the result incrementally in a format designed for
 	machine consumption.
 
+--contents <file>::
+	When <rev> is not specified, the command annotates the
+	changes starting backwards from the working tree copy.
+	This flag makes the command pretend as if the working
+	tree copy has the contents of he named file (specify
+	`-` to make the command read from the standard input).
+
 -M::
 	Detect moving lines in the file as well.  When a commit
 	moves a block of lines in a file (e.g. the original file

builtin-blame.c

@@ -18,7 +18,7 @@
 #include "cache-tree.h"
 
 static char blame_usage[] =
-"git-blame [-c] [-l] [-t] [-f] [-n] [-p] [-L n,m] [-S <revs-file>] [-M] [-C] [-C] [--contents <filename>] [commit] [--] file\n"
+"git-blame [-c] [-l] [-t] [-f] [-n] [-p] [-L n,m] [-S <revs-file>] [-M] [-C] [-C] [--contents <filename>] [--incremental] [commit] [--] file\n"
 "  -c, --compatibility Use the same output mode as git-annotate (Default: off)\n"
 "  -b                  Show blank SHA-1 for boundary commits (Default: off)\n"
 "  -l, --long          Show long commit SHA1 (Default: off)\n"