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
$ cat << EOF >> $HGRCPATH
> [extensions]
> rebase=
> [alias]
> tglog = log -G -T "{rev} '{desc}'\n"
> EOF
$ hg init
$ echo a > a; hg add a; hg ci -m a
$ echo b > b; hg add b; hg ci -m b1
$ hg up 0 -q
$ echo b > b; hg add b; hg ci -m b2 -q
$ hg tglog
@ 2 'b2'
|
| o 1 'b1'
|/
o 0 'a'
With rewrite.empty-successor=skip, b2 is skipped because it would become empty.
$ hg rebase -s 2 -d 1 --config rewrite.empty-successor=skip --dry-run
starting dry-run rebase; repository will not be changed
rebasing 2:6e2aad5e0f3c tip "b2"
note: not rebasing 2:6e2aad5e0f3c tip "b2", its destination already has all its changes
dry-run rebase completed successfully; run without -n/--dry-run to perform this rebase
With rewrite.empty-successor=keep, b2 will be recreated although it became empty.
$ hg rebase -s 2 -d 1 --config rewrite.empty-successor=keep
rebasing 2:6e2aad5e0f3c tip "b2"
note: created empty successor for 2:6e2aad5e0f3c tip "b2", its destination already has all its changes
saved backup bundle to $TESTTMP/.hg/strip-backup/6e2aad5e0f3c-7d7c8801-rebase.hg
$ hg tglog
@ 2 'b2'
|
o 1 'b1'
|
o 0 'a'