Parse username in email address

[chrizchow]

Similar to Git Plugin, getAuthor() should try to parse user using the name in the email address if the user is not found.
For example, if "chowmanchun@realtek-sg.com" cannot be found in the user database, we should try "chowmanchun" from the email address "chowmanchun@realtek-sg.com".

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[francoisferrand]

Looking at similar code from git plugin, it seems that code handles more cases (email case, creation of user...)

I think we should do the same; and I would prefer if we would reuse that code instead of copying it.