Sorting a glossary

Sort a glossary. Glossary head contains a header phrase ending with ':' and glossary body has multi line text indented with a tab.

Input

zomg:
    Multiline description
    about zomg.

abc:
    Multiline description
    about abc.

Output

abc:
    Multiline description
    about abc.

zomg:
    Multiline description
    about zomg.