changelog: introduce a 'tiprev' method
Accessing tiprev is a common need through the code base. It is usually done
using "len(changelog) -1". That form is tedious and error-prone. For example,
it will give wrong results on filtered changelog (if the unfiltered tip is
filtered).
As a result, we introduce a simple 'tiprev()' method to provide this exact
information in a nice way.
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IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false