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revset: make sort() do dumb multi-pass sorting for multiple keys (issue5218)

Our invert() function was too clever to not take length into account. I could
fix the problem by appending '\xff' as a terminator (opposite to '\0'), but
it turned out to be slower than simple multi-pass sorting.

New implementation is pretty straightforward, which just calls sort() from the
last key. We can do that since Python sort() is guaranteed to be stable. It
doesn't sound nice to call sort() multiple times, but actually it is faster.
That's probably because we have fewer Python codes in hot loop, and can avoid
heavy string and list manipulation.

  revset #0: sort(0:10000, 'branch')
  0) 0.412753
  1) 0.393254

  revset #1: sort(0:10000, '-branch')
  0) 0.455377
  1) 0.389191  85%

  revset #2: sort(0:10000, 'date')
  0) 0.408082
  1) 0.376332  92%

  revset #3: sort(0:10000, '-date')
  0) 0.406910
  1) 0.380498  93%

  revset #4: sort(0:10000, 'desc branch user date rev')
  0) 0.542996
  1) 0.486397  89%

  revset #5: sort(0:10000, '-desc -branch -user -date -rev')
  0) 0.965032
  1) 0.518426  53%

Mercurial
=========

Mercurial is a fast, easy to use, distributed revision control tool
for software developers.

Basic install:

 $ make            # see install targets
 $ make install    # do a system-wide install
 $ hg debuginstall # sanity-check setup
 $ hg              # see help

Running without installing:

 $ make local      # build for inplace usage
 $ ./hg --version  # should show the latest version

See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.