test-lib: drop PID from test-results/*.count

Each test run generates a "count" file in t/test-results that stores the number of successful, failed, etc tests. If you run "t1234-foo.sh", that file is named as "t/test-results/t1234-foo-$$.count" The addition of the PID there is serving no purpose, and makes analysis of the count files harder. The presence of the PID dates back to 2d84e9f (Modify test-lib.sh to output stats to t/test-results/*, 2008-06-08), but no reasoning is given there. Looking at the current code, we can see that other files we write to test-results (like *.exit and *.out) do _not_ have the PID included. So the presence of the PID does not meaningfully allow one to store the results from multiple runs anyway. Moreover, anybody wishing to read the *.count files to aggregate results has to deal with the presence of multiple files for a given test (and figure out which one is the most recent based on their timestamps!). The only consumer of these files is the aggregate.sh script, which arguably gets this wrong. If a test is run multiple times, its counts will appear multiple times in the total (I say arguably only because the desired semantics aren't documented anywhere, but I have trouble seeing how this behavior could be useful). So let's just drop the PID, which fixes aggregate.sh, and will make new features based around the count files easier to write. Note that since the count-file may already exist (when re-running a test), we also switch the "cat" from appending to truncating. The use of append here was pointless in the first place, as we expected to always write to a unique file. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>