genhtml: Support relative source filenames in SF keys

Some tools which generate .info files generate relative filenames for
the ‘SF’ keys. For example, nodeunit’s lcov output does. When genhtml is
run with --output-directory, it calls chdir() which breaks relative
lookup of the source files. Fix that by resolving all source filenames
to absolute paths when loading an info file, resolving any relative ones
using the info file’s path as a base.