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>

test-path-utils.c

@@ -17,5 +17,10 @@ int main(int argc, char **argv)
 		}
 	}
 
+	if (argc == 4 && !strcmp(argv[1], "longest_ancestor_length")) {
+		int len = longest_ancestor_length(argv[2], argv[3]);
+		printf("%d\n", len);
+	}
+
 	return 0;
 }