dicthelpers.diff: compare against default for missing values
This is not only a bit faster, but also aligns with callers' expectations
better since we can legitimately have manifestdict's _flags set to '' instead
of unset.
hg perfmergecalculate -r .
before: ! wall 0.139582 comb 0.140000 user 0.140000 sys 0.000000 (best of 59)
after: ! wall 0.126154 comb 0.120000 user 0.120000 sys 0.000000 (best of 74)
hg perfmergecalculate -r .^
before: ! wall 0.236333 comb 0.240000 user 0.240000 sys 0.000000 (best of 36)
after: ! wall 0.212265 comb 0.210000 user 0.210000 sys 0.000000 (best of 45)