git: simplify environment save/restore logic

The only code that cares about the value of the global variable
saved_env_before_alias after the previous fix is handle_builtin()
that turns into a glorified no-op when the variable is true, so the
logic could safely be lifted to its caller, i.e. the caller can
refrain from calling it when the variable is set.

This variable tells us if save_env_before_alias() was called (with
or without matching restore_env()), but the sole caller of the
function, handle_alias(), always calls it as the first thing, so we
can consider that the variable essentially keeps track of the fact
that handle_alias() has ever been called.

It turns out that handle_builtin() and handle_alias() are called
only from one function in a way that the value of the variable
matters, which is run_argv(), and it already keeps track of the
fact that it already called handle_alias().

So we can simplify the whole thing by:

- Change handle_builtin() to always make a direct call to the
  builtin implementation it finds, and make sure the caller
  refrains from calling it if handle_alias() has ever been
  called;

- Remove saved_env_before_alias variable, and instead use the
  local "done_alias" variable maintained inside run_argv() to
  make the same decision.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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