Log REPL output in contrib/generate_precompile.jl

[pfitzseb]

This PR catches a few cases that might cause the precompile sript to deadlock and also logs the REPL output to a file, which makes debugging errors much easier.

New output:

    JULIA usr/lib/julia/sys-o.a
Generating precompile statements (debug logging in /tmp/tmpvdRWG0)... 1050 generated in  91.869680 seconds (overhead  70.290100 seconds)
    LINK usr/lib/julia/sys.so

julia on  master [!] 
λ cat /tmp/tmpvdRWG0              
julia> 2+2
4

julia> print("")

julia> @time 1+1
  0.000004 seconds (32 allocations: 2.313 KiB)
2

shell>  pwd
/home/pfitzseb/Documents/Git/julia/base

help?>  reinterpret
search: reinterpret
...

cc @KristofferC

[vtjnash]

We've rewritten it a few times since here. Mostly we avoid the REPL now, except for a few cases such as testing the UI itself and controlling Pkg.