[MERGE] Skip unnecessary work in BzrBranch._basic_push.

[John Arbash Meinel]

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Andrew Bennetts wrote:
> 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.
> 
> 

BB:approve

John
=:->

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