Optimize RunCommand by removing /bin/sh wrapper when possible

For every $(shell echo "test: $PATH") command, when SHELL is /bin/bash,
we essentially run: (each arg wrapped in [])

  [/bin/sh] [-c] [/bin/bash -c "echo \"test: \$PATH\""]

This is redundant, since we can just use SHELL, and then we don't need
to do an extra level of shell escaping either. This change makes us run
this instead:

  [/bin/bash] [-c] [echo "test: $PATH"]

If SHELL is more complicated than an absolute path to a binary, then
we'll fall back to /bin/sh.

Using the benchmark introduced in the last change, this reduces a
minimal RunCommand execution with a simple SHELL from 3.7ms to 1.3ms.

For a more complex benchmark (though less normalized), for an AOSP
Android build, this change shrinks the average time spent in $(shell)
functions from 4.5ms to 3ms.

Change-Id: I622116e33565e58bb123ee9e9bdd302616a6609c

0 files changed, 0 insertions, 0 deletions