Redirect forwards stdout to a file for the duration of a gdb command.
This is useful when inspecting very large objects that generate several pages
of output in a gdb session.

If the GDB_EXTERNAL_EDITOR environment variable is set, popup will
automatically open the generated temporary file in the provided editor.
A simple default that works is GDB_EXTERNAL_EDITOR="gnome-open". Note that
this should be a GUI editor since you would otherwise interrupt the
active gdb session.

    redirect jco 0x12345678
    redirect x/2000xg 0x12345678

Examples: 
Change-Id: I2db78112a0141427c83813d16d94a19bc2cffddf
Reviewed-on: https://chromium-review.googlesource.com/788861
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49620}