[tz] cppcheck in-efficiencies

[Paul Eggert]

Khamit Ardashev wrote:

> date/tz.cpp:755: performance: Variable 'u' is assigned in constructor body. Consider performing initialization in initialization list.
> date/date.h:7218: performance: Function parameter 'format' should be passed by reference.
> date/tz.cpp:220: performance: Function parameter 'path' should be passed by reference.
> date/tz.cpp:2417: performance: Function parameter 'format' should be passed by reference.

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