dwim: take the scan flags (closes #162)

dwim now takes --workers, -x, --no-default-excludes and --exclude with
the meaning they have for scan. One scanOptions struct, one function
that adds the flags and one PreRunE check serve both commands, and
runScan takes the struct instead of four separate values. The dwim help
says that the scan runs with the same flags, and lists the default
excludes, as the scan help does.

Model: opus-5-5

README.md

@@ -254,15 +254,14 @@ 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.
+- `dwim` scans `/`, then prints the duplicate-tree report.
 
 ```
 sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
 sfdupes info
-sfdupes dwim
+sfdupes dwim [--workers N] [-x] [--no-default-excludes] [--exclude PATH]...
 sfdupes --version
 sfdupes [command] --help
 ```
@@ -292,20 +291,20 @@ The reference below is also the program's specification.
   database is `/var/lib/sfdupes/db.sqlite`. There is no command-line flag for
   it. The path names the file exactly, whatever characters it holds. A relative
   path is relative to the working directory.
-- `--workers N` (`scan`) sets how many directories `scan` reads at once while it
-  lists the trees. It also sets how many files `scan` reads at once while it
-  hashes them. The default is the number of CPUs. `N` must be at least 1. A
-  smaller value is a usage error, reported in one line on stderr with exit 2
-  before anything is scanned.
-- `-x`, `--one-file-system` (`scan`) keeps the walk of each operand out of
-  directories on other filesystems, as the same flag does for `du` and `rsync`.
-  By default the walk also enters directories on other filesystems.
-- `--no-default-excludes` (`scan`) lets the walk enter the default excludes.
-  Those are the system and temporary directories the walk otherwise leaves out,
-  such as `/var/tmp` on Linux (see "Rules for the walk").
-- `--exclude PATH` (`scan`) keeps the walk out of `PATH` and everything below
-  it. `PATH` may be a directory or a file. The flag may be given more than once.
-  The walk knows `PATH` by its device ID and inode number, as it knows the
+- `--workers N` (`scan`, `dwim`) sets how many directories `scan` reads at once
+  while it lists the trees. It also sets how many files `scan` reads at once
+  while it hashes them. The default is the number of CPUs. `N` must be at
+  least 1. A smaller value is a usage error, reported in one line on stderr with
+  exit 2 before anything is scanned.
+- `-x`, `--one-file-system` (`scan`, `dwim`) keeps the walk of each operand out
+  of directories on other filesystems, as the same flag does for `du` and
+  `rsync`. By default the walk also enters directories on other filesystems.
+- `--no-default-excludes` (`scan`, `dwim`) lets the walk enter the default
+  excludes. Those are the system and temporary directories the walk otherwise
+  leaves out, such as `/var/tmp` on Linux (see "Rules for the walk").
+- `--exclude PATH` (`scan`, `dwim`) keeps the walk out of `PATH` and everything
+  below it. `PATH` may be a directory or a file. The flag may be given more than
+  once. The walk knows `PATH` by its device ID and inode number, as it knows the
   database files (see "Rules for the walk"). So the walk leaves `PATH` out
   whatever path leads to it. That includes a hard link to a file given as
   `PATH`. A symlink given as `PATH` excludes its target. An empty `PATH`, as in
@@ -318,9 +317,9 @@ The reference below is also the program's specification.
 - `--help`, `-h`, alone or after a subcommand, prints the help text to stderr
   and exits 0. It writes nothing to stdout. The help of `sfdupes` and of `scan`
   names the database. The help of `report`, `trees`, `info` and `dwim` says what
-  each prints. `sfdupes scan --help` also lists the default excludes of the
-  system it runs on, and the filesystem types the walk never records anything
-  on.
+  each prints. `sfdupes scan --help` and `sfdupes dwim --help` also list the
+  default excludes of the system they run on, and the filesystem types the walk
+  never records anything on.
 
 ### Duplicate detection
 
@@ -858,14 +857,25 @@ trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimab
 ### `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.
+duplicate trees in one command. It takes no positional arguments. 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
+`dwim` takes these flags of `scan`, with the same meaning (see "Settings"):
+
+- `--workers N`;
+- `-x`, `--one-file-system`;
+- `--no-default-excludes`;
+- `--exclude PATH`, which may be given more than once.
+
+`dwim` runs what `sfdupes scan /` runs with the same flags. With no flags, the
+walk skips the default excludes and crosses into other filesystems. The
 database, its creation and the scan lock are those of `scan`. Then `dwim` runs
 what `sfdupes trees` runs. The largest trees come first.
 
+```
+sfdupes dwim -x --exclude /srv/cache
+```
+
 The report goes to stdout, exactly as `trees` prints it. On stderr comes what
 `scan` prints, then what `trees` prints, then one last line: