fix(test_common_git): add file `flush` call
fix(test_common_git): add file `flush` call

This simply adds a `flush` call after a `write` on a temp file. The
problem was that the file wasn't actually being written to without the
`flush`.

For some reason, this test was succeeding on most machines, but failing
on mine.

larsbutler/simpl,ziadsawalha/simpl,checkmate/simpl,samstav/simpl,ryandub/simpl