tests/test-ssh-repoerror.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

#require unix-permissions no-root no-windows no-rhg

XXX-RHG this test hangs if `hg` is really `rhg`. This was hidden by the use of
`alias hg=rhg` by run-tests.py. With such alias removed, this test is revealed
buggy. This need to be resolved sooner than later.

repository itself is non-readable
---------------------------------

  $ hg init no-read
  $ hg id ssh://user@dummy/no-read
  000000000000
  $ chmod a-rx no-read

  $ hg id ssh://user@dummy/no-read
  remote: abort: Permission denied: *$TESTTMP/no-read/.hg* (glob)
  abort: no suitable response from remote hg
  [255]

special case files are visible, but unreadable
----------------------------------------------

This is "similar" to the test above, but the directory is "traversable". This
seems an unexpected case in real life, but we test it anyway.

  $ hg init other
  $ hg id ssh://user@dummy/other
  000000000000
  $ for item in `find other | sort -r` ; do
  >     chmod a-r $item
  > done

  $ hg id ssh://user@dummy/other
  remote: abort: Permission denied: '$TESTTMP/other/.hg/requires'
  abort: no suitable response from remote hg
  [255]

directory toward the repository is read only
--------------------------------------------

  $ mkdir deep
  $ hg init deep/nested

  $ hg id ssh://user@dummy/deep/nested
  000000000000

  $ chmod a-rx deep

  $ hg id ssh://user@dummy/deep/nested
  remote: abort: Permission denied: *$TESTTMP/deep/nested/.hg* (glob)
  abort: no suitable response from remote hg
  [255]

repository has wrong requirement
--------------------------------

  $ hg init repo-future
  $ hg id ssh://user@dummy/repo-future
  000000000000
  $ echo flying-car >> repo-future/.hg/requires
  $ hg id ssh://user@dummy/repo-future
  remote: abort: repository requires features unknown to this Mercurial: flying-car
  remote: (see https://mercurial-scm.org/wiki/MissingRequirement for more information)
  abort: no suitable response from remote hg
  [255]