Update unpack-objects usage and documentation.

It long supported -q flag to suppress progress meter without
properly being documented.

Documentation/git-unpack-objects.txt

@@ -9,7 +9,7 @@ git-unpack-objects - Unpack objects from a packed archive.
 
 SYNOPSIS
 --------
-'git-unpack-objects' < pack-file
+'git-unpack-objects' [-q] <pack-file
 
 
 DESCRIPTION
@@ -18,6 +18,12 @@ Reads a packed archive (.pack) from the standard input, and
 expands the objects contained in the pack into "one-file
 one-object" format in $GIT_OBJECT_DIRECTORY.
 
+OPTIONS
+-------
+-q::
+	The command usually shows percentage progress.  This
+	flag suppresses it.
+
 
 Author
 ------

unpack-objects.c

@@ -6,7 +6,7 @@
 #include <sys/time.h>
 
 static int dry_run, quiet;
-static const char unpack_usage[] = "git-unpack-objects < pack-file";
+static const char unpack_usage[] = "git-unpack-objects [-q] < pack-file";
 
 /* We always read in 4kB chunks. */
 static unsigned char buffer[4096];