tests/test-empty-file.out

findrenames: Optimise "addremove -s100" by matching files by their SHA1 hashes.

We speed up 'findrenames' for the usecase when a user specifies they
want a similarity of 100% by matching files by their exact SHA1 hash
value. This reduces the number of comparisons required to find exact
matches from O(n^2) to O(n).

While it would be nice if we could just use mercurial's pre-calculated
SHA1 hash for existing files, this hash includes the file's ancestor
information making it unsuitable for our purposes. Instead, we calculate
the hash of old content from scratch.

The following benchmarks were taken on the current head of crew:

addremove 100% similarity:
  rm -rf *; hg up -C; mv tests tests.new
  hg --time addremove -s100 --dry-run

  before:  real 176.350 secs (user 128.890+0.000 sys 47.430+0.000)
  after:   real   2.130 secs (user   1.890+0.000 sys  0.240+0.000)

addremove 75% similarity:
  rm -rf *; hg up -C; mv tests tests.new; \
      for i in tests.new/*; do echo x >> $i; done
  hg --time addremove -s75  --dry-run

  before: real 264.560 secs (user 215.130+0.000 sys 49.410+0.000)
  after:  real 218.710 secs (user 172.790+0.000 sys 45.870+0.000)

0 files updated, 0 files merged, 1 files removed, 0 files unresolved
created new head
changeset:   2:62ec0e86d1e5
tag:         tip
parent:      0:567dde5e6e98
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     add empty3

changeset:   1:41ab7b321727
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     add empty2

1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
M empty2
b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty1
b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty2
b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty3