Use SIGTERM on the process group when aborting

After reading the curious Git repository corruption report in
https://gitlab.com/gitlab-org/gitlab-ce/issues/2336 (where
gitlab-git-http-server was NOT being used, for the record), I feel
inclined to be a little more cautious and use SIGTERM instead of
SIGKILL to terminate the git-xxx-pack process group in case of
aborted requests.

Also discussed in
https://gitlab.com/gitlab-org/gitlab-git-http-server/issues/1 .

githandler.go

@@ -276,8 +276,8 @@ func cleanUpProcessGroup(cmd *exec.Cmd) {
 
 	process := cmd.Process
 	if process != nil && process.Pid > 0 {
-		// Send SIGKILL (kill -9) to the process group of cmd
-		syscall.Kill(-process.Pid, syscall.SIGKILL)
+		// Send SIGTERM to the process group of cmd
+		syscall.Kill(-process.Pid, syscall.SIGTERM)
 	}
 
 	// reap our child process