More robust Git user configuration

[tkf]

Currently, BenchmarkCI.jl fails in PkgEval with

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: unable to auto-detect email address (got 'pkgeval@875c69d6f5c8.(none)')
updating: Error During Test at /home/pkgeval/.julia/packages/BenchmarkCI/jXf18/test/test_updating.jl:25
  Got exception outside of a @test
  failed process: Process(`git commit --allow-empty-message --message ''`, ProcessExited(128)) [128]
  
  Stacktrace:
    [1] pipeline_error
      @ ./process.jl:525 [inlined]
    [2] run(::Cmd; wait::Bool)
      @ Base ./process.jl:440
    [3] run(::Cmd)
      @ Base ./process.jl:438
    [4] (::BenchmarkCI.GitUtils.var"#7#14"{String, String})()
      @ BenchmarkCI.GitUtils ~/.julia/packages/BenchmarkCI/jXf18/src/gitutils.jl:55
    [5] cd(f::BenchmarkCI.GitUtils.var"#7#14"{String, String}, dir::String)
      @ Base.Filesystem ./file.jl:104
    [6] (::BenchmarkCI.GitUtils.var"#2#8"{Nothing, String, Main.TestBenchmarkCI.TestUpdating.var"#4#9", String, String})(tmpd::String)
      @ BenchmarkCI.GitUtils ~/.julia/packages/BenchmarkCI/jXf18/src/gitutils.jl:48
    [7] mktempdir(fn::BenchmarkCI.GitUtils.var"#2#8"{Nothing, String, Main.TestBenchmarkCI.TestUpdating.var"#4#9", String, String}, parent::String; prefix::String)
      @ Base.Filesystem ./file.jl:709
    [8] mktempdir
      @ ./file.jl:707 [inlined]
    [9] #updating#1
      @ ~/.julia/packages/BenchmarkCI/jXf18/src/gitutils.jl:16 [inlined]
   [10] updating
      @ ~/.julia/packages/BenchmarkCI/jXf18/src/gitutils.jl:16 [inlined]
   [11] (::Main.TestBenchmarkCI.TestUpdating.var"#3#8")(dir::String)
      @ Main.TestBenchmarkCI.TestUpdating ~/.julia/packages/BenchmarkCI/jXf18/test/test_updating.jl:32
   [12] mktempdir(fn::Main.TestBenchmarkCI.TestUpdating.var"#3#8", parent::String; prefix::String)
      @ Base.Filesystem ./file.jl:709
   [13] mktempdir(fn::Function, parent::String)
      @ Base.Filesystem ./file.jl:707
   [14] top-level scope
      @ ~/.julia/packages/BenchmarkCI/jXf18/test/test_updating.jl:26
   [15] top-level scope
      @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1113
   [16] top-level scope
      @ ~/.julia/packages/BenchmarkCI/jXf18/test/test_updating.jl:26
   [17] include(mod::Module, _path::String)
      @ Base ./Base.jl:376
   [18] include(x::String)
      @ Main.TestBenchmarkCI ~/.julia/packages/BenchmarkCI/jXf18/test/runtests.jl:1
   [19] top-level scope
      @ ~/.julia/packages/BenchmarkCI/jXf18/test/runtests.jl:7
   [20] top-level scope
      @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1188
   [21] include(fname::String)
      @ Base.MainInclude ./client.jl:444
   [22] top-level scope
      @ none:6
   [23] eval(m::Module, e::Any)
      @ Core ./boot.jl:345
   [24] exec_options(opts::Base.JLOptions)
      @ Base ./client.jl:261
   [25] _start()
      @ Base ./client.jl:485

https://github.com/JuliaCI/NanosoldierReports/blob/843cc0c9eecf24dce965f484077aa7123b39e9eb/pkgeval/by_date/2020-09/09/logs/BenchmarkCI/1.6.0-DEV-08486888ba.log#L1683-L1696