[MERGE] Skip unnecessary work in BzrBranch._basic_push.

[Andrew Bennetts]

This skips a bunch of unnecessary work in BzrBranch._basic_push for the
relatively common case of pushing 0 new revisions.  It also avoids doing
unnecessary work if there are no tags in the source branch.

This cuts the number of HPSS round trips to a constant 8 for that case.
Previously on bzr.dev it took about 3x as many (or worse, if you had lots of
pack files!).

This improvement really deserves to be enforced with some of those effort tests
I've been promising, but in the meantime here it is with no automated tests.

-Andrew.

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