Add sample commands for git-shell

Provide a 'list' command to view available bare repositories ending in
.git and a 'help command to display usage.  Also add documentation in
a README

Signed-off-by: Greg Brockman <gdb@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/git-shell-commands/README

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+Sample programs callable through git-shell.  Place a directory named
+'git-shell-commands' in the home directory of a user whose shell is
+git-shell.  Then anyone logging in as that user will be able to run
+executables in the 'git-shell-commands' directory.
+
+Provided commands:
+
+help: Prints out the names of available commands.  When run
+interactively, git-shell will automatically run 'help' on startup,
+provided it exists.
+
+list: Displays any bare repository whose name ends with ".git" under
+user's home directory.  No other git repositories are visible,
+although they might be clonable through git-shell.  'list' is designed
+to minimize the number of calls to git that must be made in finding
+available repositories; if your setup has additional repositories that
+should be user-discoverable, you may wish to modify 'list'
+accordingly.

contrib/git-shell-commands/help

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+#!/bin/sh
+
+if tty -s
+then
+	echo "Run 'help' for help, or 'exit' to leave.  Available commands:"
+else
+	echo "Run 'help' for help.  Available commands:"
+fi
+
+cd "$(dirname "$0")"
+
+for cmd in *
+do
+	case "$cmd" in
+	help) ;;
+	*) [ -f "$cmd" ] && [ -x "$cmd" ] && echo "$cmd" ;;
+	esac
+done

contrib/git-shell-commands/list

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+#!/bin/sh
+
+print_if_bare_repo='
+	if "$(git --git-dir="$1" rev-parse --is-bare-repository)" = true
+	then
+		printf "%s\n" "${1#./}"
+	fi
+'
+
+find -type d -name "*.git" -exec sh -c "$print_if_bare_repo" -- \{} \; -prune 2>/dev/null