"-f" option doesn't correctly upload the (single) tracefile

[rhaschke]

We are using the proposed method for C++ w/ cmake to generate a tracefile with lcov and uploading it with the -f option.
While locally, the generated tracefile produces a nice coverage report via genhtml, the report uploaded to codecov.io refers to files not present in the tracefile at all. I'm puzzled why this doesn't work. Here is the corresponding Travis build, starting coverage generation and uploading in line 1617.

[rhaschke]

The solution was to add the option -R <base repo> to indicate the base source folder.