[CD] support drive letters on Cygwin

The Cygwin platform supports DOS style drive-letter paths such
as "C:\\dir", even though the preferred form is a POSIX-style
"/cygdrive/c/dir".  This can be seen by doing things such as
chdir("c:") (which succeeds) followed by getcwd(NULL, 0) (which
returns the normalized "/cygdrive/c").  However, dash was trying
to perform local manipulations on the argument to 'cd' prior to
calling into libc, in order to update the state of $PWD and
friends; these manipulations were assuming that the user meant
to change to a relative subdirectory of the current location,
as in './c:', instead of honoring the drive letter.  None of
the other dash builtins take a filename and manipulate it to
affect shell state (some, like 'test', take a file name, but as
stat("c:") works just fine, there is no need to normalize).

This patch has no impact outside of cygwin; on cygwin, it takes
advantage of a native function call to canonicalize any
incoming name into preferred form before updating shell state.

Pre-patch:
$ dash -c 'cd c: && echo $PWD'
dash: 1: cd: can't cd to c:

Post-patch:
$ dash -c 'cd c: && echo $PWD'
/cygdrive/c

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 4 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 5f033df..2155764 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2014-10-13  Eric Blake <eblake@redhat.com>
+
+	* cd: support drive letters on Cygwin.
+
 2014-10-08  Herbert Xu <herbert@gondor.apana.org.au>
 
 	* Split unquoted $@/$* correctly when IFS is set but empty.