tests: Use `command -v` instead of third-party `which` program by mgorny · Pull Request #1525 · gitpython-developers/GitPython

@mgorny

Use `command -v` to locate the git executable instead of `which`.
The former is guaranteed to always be available according to POSIX,
while which(1) is a redundant third-party tool that is slowly being
phased out from Linux distributions.  In particular, Gentoo no longer
installs it by default.