Fix more inconsistent behaviour with symlinks in paths.

Both redo and minimal/do were doing slightly weird things with
symlinked directories, especially when combined with "..".  For
example, if x is a link to ., then x/x/x/x/../y should resolve to
"../y", which is quite non-obvious.

Added some tests to make sure this stays fixed.

t/105-sympath/.gitignore

@@ -0,0 +1,5 @@
+/*.dyn
+/src
+/x
+/y
+

t/105-sympath/all.do

@@ -0,0 +1,31 @@
+redo-ifchange ../flush-cache
+rm -f src
+: >src
+
+for iter in 10 20; do
+	rm -rf y
+	rm -f x *.dyn static
+	mkdir y
+	: >y/static
+	ln -s . y/x
+	../flush-cache
+
+	(
+		cd y/x/x/x/x/x
+		IFS=$(printf '\n')
+		redo-ifchange static x/x/x/static $PWD/static \
+			$(/bin/pwd)/static /etc/passwd
+		redo-ifchange $PWD/../static 2>/dev/null && exit 35
+		redo-ifchange 1.dyn x/x/x/2.dyn $PWD/3.dyn \
+			 $PWD/../4.dyn $(/bin/pwd)/5.dyn
+	)
+	[ -e y/1.dyn ] || exit $((iter + 1))
+	[ -e y/2.dyn ] || exit $((iter + 2))
+	[ -e y/3.dyn ] || exit $((iter + 3))
+	[ -e 4.dyn ]   || exit $((iter + 4))
+	[ -e y/5.dyn ] || exit $((iter + 5))
+
+	# Second iteration won't work in minimal/do since it only ever
+	# builds things once.
+	. ../skip-if-minimal-do.sh
+done

t/105-sympath/clean.do

@@ -0,0 +1,2 @@
+rm -rf y
+rm -f src x *.dyn *~ .*~

t/105-sympath/default.dyn.do

@@ -0,0 +1,2 @@
+redo-ifchange src
+echo dynamic >$3