Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database and whether its path is the default or comes
from SFDUPES_DATABASE; scan also says when the file is new. report and
trees print it before opening the database, so a missing-database error
follows it. A relative path gets the working directory put in front of
it, with its symlinks resolved as the kernel reports it, and is not
cleaned: a ".." after a symlink names another file than the cleaned
path does. README "Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.

Model: opus-5-5

cancel_test.go

@@ -344,9 +344,10 @@ func storedPaths(t *testing.T, path string) []string {
 
 // TestRunScanInterrupted calls the scan entrypoint with a context that
 // is already cancelled, as when a signal arrives at once. It must return
-// errInterrupted promptly with its one line on stderr and nothing on
-// stdout, leave the database valid and as it was, and leave nothing in
-// the way of the next scan, which must bring the database up to date.
+// errInterrupted promptly, with its line on stderr after the one naming
+// the database and nothing on stdout, leave the database valid and as
+// it was, and leave nothing in the way of the next scan, which must
+// bring the database up to date.
 func TestRunScanInterrupted(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
@@ -389,7 +390,8 @@ func TestRunScanInterrupted(t *testing.T) {
 			err, errInterrupted)
 	}
 
-	want := "scan: interrupted after 0 files\n"
+	want := "scan: database " + path + " (from " + databaseEnv + ")\n" +
+		"scan: interrupted after 0 files\n"
 	if got := stderr()[shown:]; got != want {
 		t.Errorf("stderr = %q, want %q", got, want)
 	}