Documentation: git-apply --no-add

This is a specialized hack to help no-base merges, but other
people might find it useful, so let's document it.

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

Documentation/git-apply.txt

@@ -8,7 +8,7 @@ git-apply - Apply patch on a git index file and a work tree
 
 SYNOPSIS
 --------
-'git-apply' [--stat] [--numstat] [--summary] [--check] [--index] [--apply] [--index-info] [-z] [<patch>...]
+'git-apply' [--stat] [--numstat] [--summary] [--check] [--index] [--apply] [--no-add] [--index-info] [-z] [<patch>...]
 
 DESCRIPTION
 -----------
@@ -72,6 +72,12 @@ OPTIONS
 	patch.  Give this flag after those flags to also apply
 	the patch.
 
+--no-add::
+	When applying a patch, ignore additions made by the
+	patch.  This can be used to extract common part between
+	two files by first running `diff` on them and applying
+	the result with this option, which would apply the
+	deletion part but not addition part.
 
 Author
 ------

apply.c

@@ -27,7 +27,7 @@ static int no_add = 0;
 static int show_index_info = 0;
 static int line_termination = '\n';
 static const char apply_usage[] =
-"git-apply [--stat] [--numstat] [--summary] [--check] [--index] [--apply] [--index-info] [-z] <patch>...";
+"git-apply [--stat] [--numstat] [--summary] [--check] [--index] [--apply] [--no-add] [--index-info] [-z] <patch>...";
 
 /*
  * For "diff-stat" like behaviour, we keep track of the biggest change