tests: Use command -v instead of third-party which program

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.

Author: mgorny

Diff for: test/test_git.py

@@ -169,7 +169,7 @@ def test_refresh(self):
         self.assertRaises(GitCommandNotFound, refresh, "yada")
 
         # test a good path refresh
-        which_cmd = "where" if is_win else "which"
+        which_cmd = "where" if is_win else "command -v"
         path = os.popen("{0} git".format(which_cmd)).read().strip().split("\n")[0]
         refresh(path)