tests/test-legacy-exit-code.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

Tests that the exit code is as expected when ui.detailed-exit-code is *not*
enabled.

  $ cat >> $HGRCPATH << EOF
  > [ui]
  > detailed-exit-code=no
  > EOF

  $ hg init
  $ echo a > a
Expect exit code 0 on success
  $ hg ci -Aqm initial

  $ hg co nonexistent
  abort: unknown revision 'nonexistent'
  [255]

  $ hg co 'none()'
  abort: empty revision set
  [255]

  $ hg co 'invalid('
  hg: parse error at 8: not a prefix: end
  (invalid(
           ^ here)
  [255]

  $ hg co 'invalid('
  hg: parse error at 8: not a prefix: end
  (invalid(
           ^ here)
  [255]

  $ hg continue
  abort: no operation in progress
  [255]

  $ hg st --config a=b
  abort: malformed --config option: 'a=b' (use --config section.name=value)
  [255]

  $ echo b > a
  $ hg ci -m second
  $ echo c > a
  $ hg ci -m third
  $ hg --config extensions.rebase= rebase -r . -d 0 -q
  warning: conflicts while merging a! (edit, then use 'hg resolve --mark')
  unresolved conflicts (see 'hg resolve', then 'hg rebase --continue')
  [1]