Use os.path.commonpath in guess_source_filename

[clynamen]

gcovr/gcovr/gcov.py

Line 167 in a782972

common_dir = os.path.commonprefix([data_fname, currdir])

I ran into an error where not-existing filename where generated due to common prefix. Indeed this function will return an invalid directory in this case

data_fname=/cygdrive/my_source/my_main.cpp
currdir=/cygdrive/my_build

it will result in directory /cygdrive/my due to the fact that commonprefix return the prefix string instead of the prefix path

I think the proper function to use here is
https://docs.python.org/3/library/os.path.html#os.path.commonpath

A similar issue caused #52

[latk]

Thank you for finding that! Historically, the problem is that gcovr still supports Python 2.7 which doesn't have a commonpath() function in the standard library, we therefore have a custom implementation in the gcovr.util module.

I'll fix this right away!