[PATCH] Fix git-update-index to work with relative pathnames

In particular, it fixes the following (typical for cygwin) problem:

    $ git-update-index --chmod=-x ../wrapper/Jamfile
    fatal: git-update-index: cannot chmod -x '../wrapper/Jamfile'

Signed-off-by: Alex Riesen <redacted>
---
 builtin-update-index.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)