git-checkout: safety when coming back from the detached HEAD state.

After making commits in the detached HEAD state, if you run "git
checkout" to switch to an existing branch, you will lose your
work.  Make sure the switched-to branch is a fast-forward of the
current HEAD, or require -f when switching.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -14,6 +14,8 @@ branch=
 newbranch=
 newbranch_log=
 merge=
+LF='
+'
 while [ "$#" != "0" ]; do
     arg="$1"
     shift
@@ -164,6 +166,22 @@ associate a new branch with the wanted checkout.  Example:
   git checkout -b <new_branch_name> $arg
 "
 	fi
+elif test -z "$oldbranch" && test -n "$branch"
+then
+	# Coming back...
+	if test -z "$force"
+	then
+		mb=$(git merge-base --all $old $new) &&
+		case "$LF$mb$LF" in
+		*"$LF$old$LF"*)	: ;;
+		*)	false ;;
+		esac || {
+			echo >&2 \
+"You are not on a branch and switching to $new_name branch may lose
+your changes.  Use 'git checkout -f $new_name' if you want to."
+			exit 1;
+		}
+	fi
 fi
 
 if [ "X$old" = X ]