strip: simplify some repeated conditions
We check "if saveheads or savebases" in several places to see if we
should or have created a bundle of the changesets to apply after
truncating the revlogs. One of the conditions is actually just "if
saveheads", but since there can't be savebases without saveheads, that
is effectively the same condition. It seems simpler to check only once
and from then on see if we created the file.
<?xml version="1.0" encoding="{encoding}"?>
<rss version="2.0">
<channel>
<link>{urlbase}{url|urlescape}</link>
<language>en-us</language>