tests/test-gendoc-sv.t

perf: don't clobber startrev variable

Previously, the "startrev" argument would be ignored due to
"startrev = 0" in the benchmark function. This meant that
`hg perfrevlog` always started at revision 0.

Rename the local variable to "beginrev" so the variable does the
right thing.

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors