fix incorrect chdir while processing gcov data
fix incorrect chdir while processing gcov data

* while processing the gcov data, the gcov filepath should
  be calculated related to the getcwd(), not the root dir

Signed-off-by: Kevin Cai <c4f49ef5a44534d46de426dda18a118a2ca4debf@mozy.com>

cloudsmith-io/gcovr,thatguystone/gcovr,thatguystone/gcovr,thatguystone/gcovr,kevincon/gcovr,kevincon/gcovr,cloudsmith-io/gcovr,kevincon/gcovr,cloudsmith-io/gcovr,thatguystone/gcovr,kevincon/gcovr,cloudsmith-io/gcovr