Replace which with command -v

Some distributions such as Arch Linux no longer install `which` by
default through the base package (see
https://www.archlinux.org/news/base-group-replaced-by-mandatory-base-package-manual-intervention-required/).

The maintainers have explained why `command -v` is superior:
https://www.reddit.com/r/archlinux/comments/de1er6/arch_linux_news_base_group_replaced_by_mandatory/f2v8uhu/.

bin/dotbot

@@ -7,9 +7,9 @@
 # is useful because we don't know the name of the python binary.
 
 ''':' # begin python string; this line is interpreted by the shell as `:`
-which python  >/dev/null 2>&1 && exec python  "$0" "$@"
-which python3 >/dev/null 2>&1 && exec python3 "$0" "$@"
-which python2 >/dev/null 2>&1 && exec python2 "$0" "$@"
+command -v python  >/dev/null 2>&1 && exec python  "$0" "$@"
+command -v python3 >/dev/null 2>&1 && exec python3 "$0" "$@"
+command -v python2 >/dev/null 2>&1 && exec python2 "$0" "$@"
 >&2 echo "error: cannot find python"
 exit 1
 '''

test/tests/find-python-executable.bash

@@ -31,7 +31,7 @@ touch ~/tmp_bin/python &&
 chmod +x ~/tmp_bin/python &&
 cat >> ~/tmp_bin/python <<EOF
 #!$HOME/tmp_bin/bash
-exec $(which python)
+exec $(command -v python)
 EOF
 '