dirstate-tree: Add tree traversal/iteration
Like Python’s, Rust’s iterators are "external" in that they are driven
by a caller who calls a `next` method. This is as opposed to "internal"
iterators who drive themselves and call a callback for each item.
Writing an internal iterator traversing a tree is easy with recursion,
but internal iterators cannot rely on the call stack in that way,
they must save in an explicit object all state that they need to be
preserved across two `next` calls.
This algorithm uses a `Vec` as a stack that contains what would be
local variables on the call stack if we could use recursion.
Differential Revision: https://phab.mercurial-scm.org/D10370
$ cat >> $HGRCPATH <<EOF
> [extensions]
> convert=
> EOF
Prepare orig repo
$ hg init orig
$ cd orig
$ echo foo > foo
$ HGUSER='user name' hg ci -qAm 'foo'
$ cd ..
Explicit --authors
$ cat > authormap.txt <<EOF
> user name = Long User Name
>
> # comment
> this line is ignored
> EOF
$ hg convert --authors authormap.txt orig new
initializing destination new repository
ignoring bad line in author map file authormap.txt: this line is ignored
scanning source...
sorting...
converting...
0 foo
writing author map file $TESTTMP/new/.hg/authormap
$ cat new/.hg/authormap
user name=Long User Name
$ hg -Rnew log
changeset: 0:d89716e88087
tag: tip
user: Long User Name
date: Thu Jan 01 00:00:00 1970 +0000
summary: foo
$ rm -rf new
Implicit .hg/authormap
$ hg init new
$ mv authormap.txt new/.hg/authormap
$ hg convert orig new
ignoring bad line in author map file $TESTTMP/new/.hg/authormap: this line is ignored
scanning source...
sorting...
converting...
0 foo
$ hg -Rnew log
changeset: 0:d89716e88087
tag: tip
user: Long User Name
date: Thu Jan 01 00:00:00 1970 +0000
summary: foo