This and the subsequent patches prepare for the removal of ui.settings().
Ui will be a consumer of UserSettings (or config::Config) to make it clear
that Ui can be constructed before UserSettings is fully set up.
I ran an upgraded Clippy on the codebase. All the changes seem to be
about using variables directly in format strings instead of passing
them as separate arguments.
"jj log -p --summary" now shows summary and color-words diff, like
"hg log -p --stat".
Handling of "-p" is tricky. I first considered "-p" would turn on the default
diff output, but I found it would be confusing if "jj log -p --git" showed
both color-words and git diffs. So the default format is inserted only if
no --git nor --color-words is explicitly specified.
This allows us to generate diff in different formats. There are various ways
to achieve that:
a. build TreeDiffIterator for each format (this patch)
b. make TreeDiffIterator clonable
c. collect TreeDiffIterator and reuse the resulting vec
(a) and (b) are practically the same. (c) would be more efficient if building
and iterating TreeDiffIterator were expensive, but I don't think it is because
Tree is cached by Store. If we add $GIT_EXTERNAL_DIFF-like feature, we'll
probably need Tree objects to snapshot them to /tmp. So I chose (a).
