minimal/do: fix a bug when $PWD != $(/bin/pwd).

This can happen when $PWD contains a symlink somewhere in the path.  In
that case, "cd ..; cat x" could mean something different from "cat ../x".

Notably, this error occurs when running "./do test" if your build
directory is through a symlink.  For example, on freebsd your home
directory is /home/$USER, but /home is a symlink to /usr/home, which
triggers this problem.

Not adding tests in this commit, because when I added some tests, I
found even more symlink-related bugs, but those ones are much more
unlikely to occur.  The additional fixes+tests are in a later commit.

do

@@ -24,7 +24,7 @@ usage() {
 }
 
 mydir=$(dirname "$0")
-cd "$mydir" || die "can't find self in dir: $mydir"
+cd "$(/bin/pwd)" && cd "$mydir" || die "can't find self in dir: $mydir"
 
 args=
 while [ "$1" != "${1#-}" ]; do