Specifying --bench-on dirty is broken

[LilithHafner]

This is a great error:

shell> ../../bin/benchpkg --bench-on this-ref-does-not-exist --rev dirty,main
[ Info: Downloading package's latest benchmark script, assuming it is in benchmark/benchmarks.jl
[ Info: Downloading from this-ref-does-not-exist.
ERROR: LoadError: Did not find rev this-ref-does-not-exist in repository
...

This is probably a bug:

shell> ../../bin/benchpkg --bench-on dirty --rev dirty,main
ERROR: LoadError: MethodError: no method matching length(::Nothing)
The function `length` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  length(::ExproniconLite.JLIfElse)
   @ ExproniconLite ~/.julia/packages/ExproniconLite/nABwg/src/types.jl:118
  length(::Core.SimpleVector)
   @ Base essentials.jl:933
  length(::JSON.Parser.MemoryParserState)
   @ JSON ~/.julia/packages/JSON/93Ea8/src/Parser.jl:28
  ...

Stacktrace:
 [1] benchpkg(package_name::String; rev::String, output_dir::String, script::String, exeflags::String, add::String, tune::Bool, url::String, path::String, bench_on::String, filter::String, nsamples_load_time::Int64, dont_print::Bool)
   @ AirspeedVelocity.BenchPkg ~/.julia/packages/AirspeedVelocity/8uSQr/src/BenchPkg.jl:94
 [2] benchpkg
   @ ~/.julia/packages/AirspeedVelocity/8uSQr/src/BenchPkg.jl:53 [inlined]
 [3] command_main(ARGS::Vector{String})
   @ AirspeedVelocity.BenchPkg ~/.julia/packages/Comonicon/F3QqZ/src/codegen/julia.jl:343
 [4] command_main()
   @ AirspeedVelocity.BenchPkg ~/.julia/packages/Comonicon/F3QqZ/src/codegen/julia.jl:90
 [5] top-level scope
   @ ~/.julia/bin/benchpkg:14
in expression starting at /home/x/.julia/bin/benchpkg:14

[LilithHafner]

This, combined with #58 makes it hard to test benchmarks without committing them first.

[MilesCranmer]

The current way to do this to just use the -s,--script argument to specify the script explicitly, like benchpkg -s benchmark/benchmarks.jl. But yeah it would be nice if --bench-on dirty did the obvious thing!