Fix 'git commit -a' in a newly initialized repository

With current git:

$ git init
$ git commit -a
cp: cannot stat `.git/index': No such file or directory

Output a nice error message instead.

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-commit.sh

@@ -318,6 +318,10 @@ esac
 
 case "$all,$also" in
 t,)
+	if test ! -f "$THIS_INDEX"
+	then
+		die 'nothing to commit (use "git add file1 file2" to include for commit)'
+	fi
 	save_index &&
 	(
 		cd_to_toplevel &&