Add sfdupes info: show the database's details (closes #117)

info prints the database's path and source, the sizes of the database
file and its -wal file, when the database file last changed, the schema
version and the record counts, one per line as a name, a tab and the
value. It opens the database as report does and shows an open and a
count phase on stderr. The schema version and the counts come from one
read-only transaction, the counts from one query.

seedDatabase now closes test databases out of WAL mode, as scan does;
otherwise info's own connection creates an empty -wal file. The open
phase's name and text became constants for the linter, and the scan
command moved to newScanCommand to keep newRootCommand short enough.

Judgement call: the lines are written after the last display ends, so they skip the display's stdout writer.

Model: opus-5-5

db.go

@@ -79,8 +79,7 @@ ON CONFLICT (path) DO UPDATE SET
     content = excluded.content
 `
 
-// errNoDatabase reports a missing database file for report, trees and
+// errNoDatabase reports a missing database file for report/trees.
-// info.
 var errNoDatabase = errors.New(
 	"no database (run \"sfdupes scan\" first, or set " + databaseEnv + ")")
 
@@ -132,9 +131,9 @@ const scanParams = "_pragma=busy_timeout(10000)" +
 	"&_pragma=journal_mode(WAL)" +
 	"&_pragma=synchronous(NORMAL)"
 
-// reportParams are the connection parameters for report, trees and
+// reportParams are the connection parameters for report and trees:
-// info: read-only, with the same busy timeout. They set no journal
+// read-only, with the same busy timeout. They set no journal mode,
-// mode, because setting one is a write.
+// because setting one is a write.
 const reportParams = "mode=ro" +
 	"&_pragma=busy_timeout(10000)" +
 	"&_pragma=query_only(1)"
@@ -263,9 +262,9 @@ func closeScanDatabase(ctx context.Context, db *sql.DB, path string) {
 	_ = db.Close()
 }
 
-// openReportDatabase opens an existing database for the report, trees
+// openReportDatabase opens an existing database for the report and
-// and info subcommands. A missing database file is an error directing
+// trees subcommands. A missing database file is an error directing the
-// the user to run scan first; the schema version must match exactly.
+// user to run scan first; the schema version must match exactly.
 func openReportDatabase(ctx context.Context,
 	path string,
 ) (*sql.DB, error) {