Add sfdupes dwim: scan / then print duplicate trees (closes #154)

dwim runs what "sfdupes scan /" runs with no flags, then what
"sfdupes trees" runs, and ends with a stderr line saying that
"sfdupes trees" prints the report again. An interrupt or fatal scan
error ends it before the report, as scan ends.

Model: opus-5-5

README.md

@@ -17,12 +17,13 @@ gigabyte. A large file is never read in full. A match between two large files is
 a strong candidate, not proof.
 
 `sfdupes` is meant for multi-terabyte servers, where reading every byte of every
-file takes too long. It has four subcommands. `scan` walks the filesystem and
+file takes too long. It has five subcommands. `scan` walks the filesystem and
 records the files it finds in an SQLite database. SQLite keeps a whole database
 in one file. The database is kept between runs. A rescan reads only new and
 changed files. That makes `scan` cheap enough to run every day from `cron`, the
 system's job scheduler. `report`, `trees` and `info` read the database alone and
-can run at any time.
+can run at any time. `dwim` scans `/` and then prints the `trees` report, for a
+first look at a new machine with one command.
 
 ## Getting Started
 
@@ -247,12 +248,15 @@ In order:
 - `report` prints the file-level duplicate report from the database.
 - `trees` prints the duplicate-tree report from the database.
 - `info` prints the database's details.
+- `dwim` scans `/` with the default excludes, then prints the duplicate-tree
+  report.
 
 ```
 sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
 sfdupes info
+sfdupes dwim > dupetrees.tsv
 sfdupes --version
 sfdupes [command] --help
 ```
@@ -800,6 +804,32 @@ The summary line on stderr gives:
 trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimable
 ```
 
+### `dwim` mode
+
+`dwim`, short for "do what I mean", scans a new machine and reports its
+duplicate trees in one command. It takes no positional arguments and no flags of
+its own. Like `scan`, it should run as root, so that it can read every file.
+
+`dwim` runs what `sfdupes scan /` runs with no flags. The walk skips the default
+excludes and crosses into other filesystems. `--workers` keeps its default. The
+database, its creation and the scan lock are those of `scan`. Then `dwim` runs
+what `sfdupes trees` runs. The largest trees come first.
+
+The report goes to stdout, exactly as `trees` prints it. On stderr comes what
+`scan` prints, then what `trees` prints, then one last line:
+
+```
+dwim: run "sfdupes trees" to print this report again
+```
+
+The database keeps the scan, so `sfdupes trees` prints the same report later,
+even after the terminal is gone.
+
+`SIGINT` or `SIGTERM` stops the scan as it stops `scan` (see "Stopping a scan").
+`dwim` then prints no report and exits 1. A fatal scan error also ends it before
+the report, with the same message and exit code as `scan`. Warnings for skipped
+files do not stop it.
+
 ### `info` mode
 
 `info` takes no positional arguments and prints the database's details on
@@ -840,7 +870,7 @@ scan is writing. `info` writes its lines after the `count` display has ended.
 
 ### Database
 
-All four subcommands work on one SQLite database file.
+All five subcommands work on one SQLite database file.
 
 #### Location
 
@@ -1139,8 +1169,8 @@ showing the count it stopped at.
 ### Error handling and exit codes
 
 - `0`: success, even when files were skipped with warnings.
-- `1`: a fatal error, or a `scan` stopped by `SIGINT` or `SIGTERM`. A fatal
-  error is one of these:
+- `1`: a fatal error, or a `scan` or `dwim` stopped by `SIGINT` or `SIGTERM`. A
+  fatal error is one of these:
     - an operand that does not exist, or whose real path `scan` cannot find;
     - a list of mounted filesystems that `scan` cannot read;
     - another `scan` holding the lock (see "The scan lock");
@@ -1161,7 +1191,7 @@ showing the count it stopped at.
       `--workers abc`;
     - `scan` with no operand;
     - `--workers` below 1;
-    - `report`, `trees` or `info` with a positional argument.
+    - `report`, `trees`, `info` or `dwim` with a positional argument.
 
 A fatal error is reported on stderr, starting with `sfdupes:`. A failed write to
 stdout, such as a full disk, is reported the same way. Two cases never reach