Simplify test-lib/git
Simplify test-lib/git

The optional options argument is never used, so neither is stdout/stderr
in the resolved Promise value.  Remove the unused code and unnecessary
pify dependency.

Add a comment about get-exec-file which returns ChildProcess with
then/catch for use as a Promise in case we need it later.

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/git-branch-is,kevinoid/git-branch-is,kevinoid/git-branch-is