Documentation/shortlog: scripted users should not rely on implicit HEAD

When passed no revision arguments, ‘git shortlog’ reads a log from
stdin if and only if stdin is not a tty.  So scripts that need to
function identically when standard input is a terminal (as when run
interactively) and not (as when run through a cron job) should either
supply a log themselves or specify the desired revisions explicitly.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 git log --pretty=short | 'git shortlog' [-h] [-n] [-s] [-e] [-w]
-'git shortlog' [-n|--numbered] [-s|--summary] [-e|--email] [-w[<width>[,<indent1>[,<indent2>]]]] [<committish>...]
+'git shortlog' [-n|--numbered] [-s|--summary] [-e|--email] [-w[<width>[,<indent1>[,<indent2>]]]] <commit>...
 
 DESCRIPTION
 -----------
@@ -19,6 +19,11 @@ the first line of the commit message will be shown.
 
 Additionally, "[PATCH]" will be stripped from the commit description.
 
+If no revisions are passed on the command line and either standard input
+is not a terminal or there is no current branch, 'git shortlog' will
+output a summary of the log read from standard input, without
+reference to the current repository.
+
 OPTIONS
 -------