Use correct file name for history.xml files

Correct a tendency of the plugin to leave temporary XML files lying around
that ended up in the nar archive. The change here rearranges the code to
delete the temporary XML rather than leave it around.

This fixes issue #106.

Johannes Schindelin was the original author of the change. IT's pass, no
wierd files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Spell out XML as a String, not as hex numbers

This clarifies how we initialize the history.xml file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #114 from benson-basis/history-xml-fun

Issue #106: apply @dscho's suggested change.

Basically, it fixes the problem that the history.xml file was added
with a long semi-random number suffix and therefore could not be
used effectively, neither would it accumulate the build history.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>