git p4 test: translate windows paths for cygwin

git p4 test: translate windows paths for cygwin

Native windows binaries do not understand posix-like
path mapping offered by cygwin.  Convert paths to native
using "cygpath --windows" before presenting them to p4d.

This is done using the AltRoots mechanism of p4.  Both the
posix and windows forms are put in the client specification,
allowing p4 to find its location by native path even though
the environment reports a different PWD.

Shell operations in tests will use the normal form of $cli,
which will look like a posix path in cygwin, while p4 will
use AltRoots to match against the windows form of the working
directory.

This mechanism also handles the symlink issue that was fixed in
23bd0c9 (git p4 test: use real_path to resolve p4 client
symlinks, 2012-06-27).  Now that every p4 client view has
an AltRoots with the real_path in it, explicitly calculating
the real_path elsewhere is not necessary.

Thanks-to: Sebastian Schuberth <sschuberth@gmail.com>
Thanks-to: Johannes Sixt <j6t@kdbg.org>
fixup! git p4 test: translate windows paths for cygwin

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>