show: --ignore-missing

Instead of barfing, simply ignore bad object names seen in the
input. This is useful when reading from "git notes list" output
that may refer to objects that have already been garbage collected.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-list.txt

@@ -29,6 +29,7 @@ SYNOPSIS
 	     [ \--tags[=<pattern>] ]
 	     [ \--remotes[=<pattern>] ]
 	     [ \--glob=<glob-pattern> ]
+	     [ \--ignore-missing ]
 	     [ \--stdin ]
 	     [ \--quiet ]
 	     [ \--topo-order ]

Documentation/rev-list-options.txt

@@ -139,6 +139,10 @@ parents) and `--max-parents=-1` (negative numbers denote no upper limit).
 	is automatically prepended if missing. If pattern lacks '?', '*',
 	or '[', '/*' at the end is implied.
 
+--ignore-missing::
+
+	Upon seeing an invalid object name in the input, pretend as if
+	the bad input was not given.
 
 ifndef::git-rev-list[]
 --bisect::