Currently there are no safeguards in place to prevent us from making lots of calls to $EXTERNAL_LINTER --version (which is a relatively slow operation). Instead, we can provide a simple cacheing mechanism (using a static variable) in ArcanistExternalLinter. This mainly affects the unit tests of rARC, but there may be other situations in which this will yield performance improvements.

Before

> time ./bin/arc unit -- src/lint/linter
real    0m38.472s
user    0m35.614s
sys 0m3.536s

After

> time ./bin/arc unit -- src/lint/linter
real    0m27.364s
user    0m25.601s
sys 0m2.338s