st: Small bug with variable names in xtpoisson

[Paulo Guimaraes <guimaraes@moore.sc.edu>]

There is a small bug with reported variable names in xtpoisson.
When the lag operator is used the command does not report the variable 
name but instead it reports the name of a temporary variable.
To replicate do:

webuse ships
bys ship: gen time=_n
xtset ship time
xtpoisson accident l.service

Paulo Guimaraes

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