Add support for GIT_CEILING_DIRECTORIES

Add support for GIT_CEILING_DIRECTORIES

Make git recognize a new environment variable that prevents it from
chdir'ing up into specified directories when looking for a GIT_DIR.
Useful for avoiding slow network directories.

For example, I use git in an environment where homedirs are automounted
and "ls /home/nonexistent" takes about 9 seconds.  Setting
GIT_CEILING_DIRS="/home" allows "git help -a" (for bash completion) and
"git symbolic-ref" (for my shell prompt) to run in a reasonable time.

Signed-off-by: David Reiss <dreiss@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>