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