Support combining coverage data from different platforms/locations
I collect coverage data on both Linux and Windows as there is a fair amount of code in my system that differs between these platforms. I would therefore like to be able to use the -c option to combine this data, but of course the problem is that the same files are referred to in different ways. I'd therefore suggest the concept of an "alternative current working directory" whereby I can tell the coverage tool that a certain location is to be considered as equivalent to the local source tree. This could then also be used for example to combine data collected by different users on equivalent source, even if it wasn't cross-platform.
I enclose a patch which achieves this via a -A option on the command line (note this patch also includes the fixes for bugs 14 and 16, so is a quick way to get everything at once)