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 rep
$ cd rep
$ mkdir dir
$ touch foo dir/bar
$ hg -v addremove
adding dir/bar
adding foo
$ hg -v commit -m "add 1"
committing files:
dir/bar
foo
committing manifest
committing changelog
committed changeset 0:6f7f953567a2
$ cd dir/
$ touch ../foo_2 bar_2
$ hg -v addremove
adding dir/bar_2
adding foo_2
$ hg -v commit -m "add 2"
committing files:
dir/bar_2
foo_2
committing manifest
committing changelog
committed changeset 1:e65414bf35c5
$ cd ..
$ hg forget foo
$ hg -v addremove
adding foo
$ hg forget foo
$ hg -v addremove nonexistent
nonexistent: $ENOENT$
[1]
$ cd ..
$ hg init subdir
$ cd subdir
$ mkdir dir
$ cd dir
$ touch a.py
$ hg addremove 'glob:*.py'
adding a.py
$ hg forget a.py
$ hg addremove -I 'glob:*.py'
adding a.py
$ hg forget a.py
$ hg addremove
adding dir/a.py
$ cd ..
$ hg init sim
$ cd sim
$ echo a > a
$ echo a >> a
$ echo a >> a
$ echo c > c
$ hg commit -Ama
adding a
adding c
$ mv a b
$ rm c
$ echo d > d
$ hg addremove -n -s 50 # issue 1696
removing a
adding b
removing c
adding d
recording removal of a as rename to b (100% similar)
$ hg addremove -ns 50 --color debug
[ui.addremove.removed ui.status|removing a]
[ui.addremove.added ui.status|adding b]
[ui.addremove.removed ui.status|removing c]
[ui.addremove.added ui.status|adding d]
[ ui.status|recording removal of a as rename to b (100% similar)]
$ hg addremove -s 50
removing a
adding b
removing c
adding d
recording removal of a as rename to b (100% similar)
$ hg commit -mb
$ cp b c
$ hg forget b
$ hg addremove -s 50
adding b
adding c
$ rm c
$ hg ci -A -m "c" nonexistent
nonexistent: $ENOENT$
abort: failed to mark all new/missing files as added/removed
[255]
$ hg st
! c
$ cd ..