We'll probably add binary comparison operators at some point, but this patch
also adds size_hint.zero() method. Otherwise, we'll have to write
"if(x.upper() && x.upper() == 0, ..)" to deal with None.
The resulting "branch list" template will look like:
```
separate(", ",
if(!ref.tracking_ahead_count().zero(),
if(ref.tracking_ahead_count().exact(),
"ahead by " ++ ref.tracking_ahead_count().exact() ++ " commits",
"ahead by at least " ++ ref.tracking_ahead_count().lower() ++ " commits")),
if(!ref.tracking_behind_count().zero(),
if(ref.tracking_behind_count().exact(),
"behind by " ++ ref.tracking_behind_count().exact() ++ " commits",
"behind by at least " ++ ref.tracking_behind_count().lower() ++ " commits")),
)
```
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