git-plot: Don't use mktemp(1) to create tempfile

It complains about too few X's:

  $ ./git-plot
  mktemp: too few X's in template `git-plot.1370904740.31672.XXXXXX.tmp'
  ./git-plot: line 11: $tmpfile: ambiguous redirect

  Syntax:

    /home/sunny/bin/stpl [options] data_file

It's only here (Ubuntu 10.04.4 LTS) it complains, maybe it's too old or
this distro uses another mktemp than the other distros. Don't need it,
though. Should be enough with epoch date and PID.

3affa7d2-d221-11e2-8905-fefdb24f8e10