Documentation: document show -s

Git's diff machinery has supported a -s (silence diff output) option
as far back as v0.99~900 (Silent flag for show-diff, 2005-04-13), but
the option is only advertised in an odd corner of the git diff-tree
manual.

The main use is to retrieve basic metadata about a commit:

	git show -s rev

Explain this in the 'git log' manual and provide an example in the
'git show' examples section.  This is kind of a cop-out, since it
would be more useful to explain it in the 'git show' manual proper,
which says:

	The command takes options applicable to the git
	diff-tree command to control how the changes the
	commit introduces are shown.

	This manual page describes only the most frequently
	used options.

Fixing that is a larger task for another day.

Reported-by: Will Hall <will@gnatter.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show.txt

@@ -54,6 +54,10 @@ git show v1.0.0::
 git show v1.0.0^\{tree\}::
 	Shows the tree pointed to by the tag `v1.0.0`.
 
+git show -s --format=%s v1.0.0^\{commit\}::
+	Shows the subject of the commit pointed to by the
+	tag `v1.0.0`.
+
 git show next~10:Documentation/README::
 	Shows the contents of the file `Documentation/README` as
 	they were current in the 10th last commit of the branch

Documentation/rev-list-options.txt

@@ -146,6 +146,9 @@ options may be given. See linkgit:git-diff-files[1] for more options.
 -t::
 
 	Show the tree objects in the diff output. This implies '-r'.
+
+-s::
+	Suppress diff output.
 endif::git-rev-list[]
 
 Commit Limiting