tests/test-close-head.t

encoding: fix trim() to be O(n) instead of O(n^2)

`encoding.trim()` iterated over the possible lengths smaller than the
input and created a slice for each. It then calculated the column
width of the result, which is of course O(n), so the overall algorithm
was O(n). This patch rewrites it to iterate over the unicode
characters, keeping track of the length so far. Also, the old
algorithm started from the end of the string, which made it much worse
when the input is large and the limit is small (such as the typical 72
we pass to it).

You can time it by running something like this:

```
time python3 -c 'from mercurial.utils import stringutil; print(stringutil.ellipsis(b"0123456789" * 1000, 5))'
```

That drops from 4.05 s to 83 ms with this patch (and most of that is
of course startup time).

Differential Revision: https://phab.mercurial-scm.org/D12089

  $ hg init test-content
  $ cd test-content
  $ hg debugbuilddag '+2*2*3*4+7'
  $ hg bookmark -r 1 @
  $ hg log -G --template '{rev}:{node|short}'
  o  11:1d876b1f862c
  |
  o  10:ea5f71948eb8
  |
  o  9:f1b0356d867a
  |
  o  8:e8d1253fb0d7
  |
  o  7:d423bbba4459
  |
  o  6:a2f58e9c1e56
  |
  o  5:3a367db1fabc
  |
  o  4:e7bd5218ca15
  |
  | o  3:6100d3090acf
  |/
  | o  2:fa942426a6fd
  |/
  | o  1:66f7d451a68b
  |/
  o  0:1ea73414a91b
  
  $ hg --config extensions.closehead= close-head -m 'Not a head' 0 1
  abort: revision is not an open head: 0
  [255]
  $ hg --config extensions.closehead= close-head -m 'Not a head' -r 0 1
  abort: revision is not an open head: 0
  [255]
  $ hg id
  000000000000
  $ hg --config extensions.closehead= close-head -m 'Close old heads' -r 1 2
  $ hg id
  000000000000
  $ hg bookmark
     @                         1:66f7d451a68b
  $ hg heads
  changeset:   11:1d876b1f862c
  user:        debugbuilddag
  date:        Thu Jan 01 00:00:11 1970 +0000
  summary:     r11
  
  changeset:   3:6100d3090acf
  parent:      0:1ea73414a91b
  user:        debugbuilddag
  date:        Thu Jan 01 00:00:03 1970 +0000
  summary:     r3
  
  $ hg --config extensions.closehead= close-head -m 'Close more old heads' -r 11
  $ hg heads
  changeset:   3:6100d3090acf
  parent:      0:1ea73414a91b
  user:        debugbuilddag
  date:        Thu Jan 01 00:00:03 1970 +0000
  summary:     r3
  
  $ hg --config extensions.closehead= close-head -m 'Not a head' 0
  abort: revision is not an open head: 0
  [255]
  $ hg --config extensions.closehead= close-head -m 'Already closed head' 1
  abort: revision is not an open head: 1
  [255]

  $ hg init ../test-empty
  $ cd ../test-empty
  $ hg debugbuilddag '+1'
  $ hg log -G --template '{rev}:{node|short}'
  o  0:1ea73414a91b
  
  $ hg --config extensions.closehead= close-head -m 'Close initial revision' 0
  $ hg heads
  [1]