Fix incorrect behavior of 'cd ../.foo' (#46)

The cd builtin was removing '.' from directory names when combined
with a preceding '../', which caused commands like 'cd ../.local'
to become 'cd ../local'. This patch fixes the problem by limiting
the extra handling to leading '..'. The bugfix comes from ksh93v-
2013-10-10-alpha, although this version is a shortened patch from
Solaris (as ksh93v- refactored a decent amount of the code for the
cd builtin).

src/cmd/ksh93/bltins/cd_pwd.c:
- cd should only check for leading '..', as trying to handle a lone
  '.' only causes problems.

src/cmd/ksh93/tests/builtins.sh:
- Add a regression test for this problem based on the test present in
  ksh93v- 2013-10-10-alpha.

Patch from Solaris:
860d27f/components/ksh93/patches/270-23319761.patch

src/cmd/ksh93/tests/builtins.sh

@@ -717,5 +717,14 @@ chmod is $(whence -p chmod)
 chmod is a tracked alias for $(whence -p chmod)"
 [[ $actual == $expected ]] || err_exit '`whence -a` does not work correctly with tracked aliases'
 
+# ======
+# 'cd ../.foo' should not exclude the '.' in '.foo'
+(
+	cd "$tmp"
+	mkdir foo .bar
+	cd foo
+	cd ../.bar
+) || err_exit 'cd ../.bar when ../.bar exists should not fail'
+
 # ======
 exit $((Errors<125?Errors:125))