Fix a bug in coverage reporting.

The directory from where the source files are sought when producing a
coverage report in text or XML format should not be the build directory
but the source directory instead.

mesonbuild/backend/ninjabackend.py

@@ -431,12 +431,12 @@ int dummy;
         if gcovr_exe:
             added_rule = True
             elem = NinjaBuildElement(self.all_outputs, 'coverage-xml', 'CUSTOM_COMMAND', '')
-            elem.add_item('COMMAND', [gcovr_exe, '-x', '-r', self.environment.get_build_dir(),\
+            elem.add_item('COMMAND', [gcovr_exe, '-x', '-r', self.environment.get_source_dir(),\
                                       '-o', os.path.join(self.environment.get_log_dir(), 'coverage.xml')])
             elem.add_item('DESC', 'Generating XML coverage report.')
             elem.write(outfile)
             elem = NinjaBuildElement(self.all_outputs, 'coverage-text', 'CUSTOM_COMMAND', '')
-            elem.add_item('COMMAND', [gcovr_exe, '-r', self.environment.get_build_dir(),\
+            elem.add_item('COMMAND', [gcovr_exe, '-r', self.environment.get_source_dir(),\
                                       '-o', os.path.join(self.environment.get_log_dir(), 'coverage.txt')])
             elem.add_item('DESC', 'Generating text coverage report.')
             elem.write(outfile)