Porcelain scripts: Rewrite cryptic "needs update" error message

Although Git interally has the facility to differentiate between
porcelain and plubmbing commands and appropriately print errors,
several shell scripts invoke plubming commands triggering cryptic
plumbing errors to be displayed on a porcelain interface. This patch
replaces the "needs update" message in git-pull and git-rebase, when
`git update-index` is run, with a more friendly message.

Reported-by: Joshua Jensen <jjensen@workspacewhiz.com>
Reported-by: Thore Husfeldt <thore.husfeldt@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -145,6 +145,35 @@ require_work_tree () {
 	die "fatal: $0 cannot be used without a working tree."
 }
 
+require_clean_work_tree () {
+	git rev-parse --verify HEAD >/dev/null || exit 1
+	git update-index -q --ignore-submodules --refresh
+	err=0
+
+	if ! git diff-files --quiet --ignore-submodules
+	then
+		echo >&2 "Cannot $1: You have unstaged changes."
+		err=1
+	fi
+
+	if ! git diff-index --cached --quiet --ignore-submodules HEAD --
+	then
+		if [ $err = 0 ]
+		then
+		    echo >&2 "Cannot $1: Your index contains uncommitted changes."
+		else
+		    echo >&2 "Additionally, your index contains uncommitted changes."
+		fi
+		err=1
+	fi
+
+	if [ $err = 1 ]
+	then
+		test -n "$2" && echo >&2 "$2"
+		exit 1
+	fi
+}
+
 get_author_ident_from_commit () {
 	pick_author_script='
 	/^author /{