trees: show its phases as N of M, with progress, elapsed time and ETA (closes #113)

trees now runs in four phases, open, read, group and write, each under
the progress display scan uses, until the summary line. open covers a
wait on a busy database. read counts the records and reads them in one
read-only transaction, so the two agree during a scan; group counts the
directories, and write the groups. The rows reach stdout through a
writer that clears the display and holds its lock while writing, so no
row shares a line with the display on a shared terminal. Each rate
names what its phase counts.

Judgement call: opening the database is a phase of its own, so there are four phases, not the three the issue sketches.

Model: opus-5-5

README.md

@@ -695,6 +695,12 @@ Processing:
 - Order groups by total tree size descending, tie-broken by `first` path
   ascending. Output must be fully deterministic for a given input.
 
+`trees` runs in four phases, each with its own progress display on stderr (see
+"Progress"): `open` opens the database; `read` counts the records, then reads
+them in path order, building the hierarchy and computing the digests as it goes;
+`group` groups the directories by digest and picks the maximal groups; `write`
+writes the groups to stdout.
+
 #### Trees output format
 
 TSV on stdout: a header line, then one row per duplicate tree (N-1 rows for a
@@ -715,13 +721,16 @@ units.
 
 ### Progress
 
-Use the progress-bar library for all scan progress; rendering in the style of
-`pv` is the model. All progress goes to stderr.
+Use the progress-bar library for all progress of `scan` and `trees`; rendering
+in the style of `pv` is the model. All progress goes to stderr.
 
-A scan must always show that it is working and not hung. Each phase gets its own
-display from the moment it starts, naming the phase and its place among the five
-(`[1/5] load`, `[2/5] walk`, `[3/5] hash`, `[4/5] update`, `[5/5] content`) and
-showing its progress, its elapsed time and its estimated time remaining (ETA):
+`scan` and `trees` must always show that they are working and not hung. Each
+phase gets its own display from the moment it starts, naming the phase and its
+place among the command's phases and showing its progress, its elapsed time and
+its estimated time remaining (ETA).
+
+A scan has five phases (`[1/5] load`, `[2/5] walk`, `[3/5] hash`,
+`[4/5] update`, `[5/5] content`):
 
 - `load` counts the records first, then reads them against that count. While the
   count runs, the display says it is counting the records.
@@ -738,12 +747,23 @@ showing its progress, its elapsed time and its estimated time remaining (ETA):
   and ETA, start there. The files to read then get a display of their own, with
   their count as its total.
 
+`trees` has four phases (`[1/4] open`, `[2/4] read`, `[3/4] group`,
+`[4/4] write`, see "`trees` mode"):
+
+- `open` has no total. While it opens the database, waiting on a busy one
+  included, the display says it is opening the database.
+- `read` counts the records first, then reads them against that count. While the
+  count runs, the display says it is counting the records.
+- `group` has the directories `read` found as its total, and `write` the
+  duplicate-tree groups.
+
 Each display is one line holding:
 
 - the phase and its place among the phases
 - a `[m/n] x%` display (items processed / total items, percent); before a phase
   has a total, the items processed, or what the phase is waiting for
-- current rate (items/s)
+- current rate (items/s), naming the items the phase counts: files, records,
+  directories or groups
 - elapsed time
 - estimated time remaining, or `unknown`
 
@@ -756,13 +776,13 @@ Example shape (exact layout is flexible, content is not):
 
 Additional requirements:
 
-- On a terminal, a phase with an exact total (every phase but `walk`, once it
-  has counted what it will do) is drawn as its line followed by the progress-bar
-  library's bar, 10 columns wide, which shows the percent in place of the line's
-  own. Any other phase, and a phase whose bar has no room, is drawn as the
-  library's spinner followed by its line. Each display is drawn again several
-  times a second, so its elapsed time keeps moving while a phase waits for its
-  next item.
+- On a terminal, a phase with an exact total (every phase but `scan`'s `walk`
+  and `trees`' `open`, once it has counted what it will do) is drawn as its line
+  followed by the progress-bar library's bar, 10 columns wide, which shows the
+  percent in place of the line's own. Any other phase, and a phase whose bar has
+  no room, is drawn as the library's spinner followed by its line. Each display
+  is drawn again several times a second, so its elapsed time keeps moving while
+  a phase waits for its next item.
 - On a terminal no line reaches the terminal's last column, however narrow the
   terminal is. The phase and its place, the progress, the elapsed time and the
   ETA are kept whole: a line too wide for the terminal leaves out its rate first
@@ -781,11 +801,11 @@ Additional requirements:
   goroutine counts the items, and each display draws them from a goroutine of
   its own.
 - A warning printed during a phase always lands on a line of its own, never
-  inside the progress display.
+  inside the progress display. So does every line written to stdout when stdout
+  is the same terminal, as in a plain `sfdupes trees`.
 - A phase that stops short of its total, as an interrupted one does, is left
   showing the count it stopped at rather than filled up.
-- `report` and `trees` modes need no progress display, only their stderr
-  summaries.
+- `report` mode needs no progress display, only its stderr summary.
 
 ### Error handling and exit codes