Skip removed paths silently and fix the progress line (closes #133)

A path removed before the scan reaches it, in the walk or the hash
phase, gets no warning, no skipped count and no record. If the first of
several hard links is gone, the next is read.

On a terminal every display is the library's spinner, then the line. A
phase with an exact total ends the padded line with its percent and a
bar at a fixed column. sfdupes draws that bar: the library shows no
spinner beside its own.

The time remaining is labelled remain, not eta.

Judgement call: an operand removed after the scan starts is passed over
silently too.
Deviation: README's 80-column promise for stalled lines is narrowed to
its example.

Model: opus-5-5

trees_test.go

@@ -13,7 +13,7 @@ import (
 // treesOpenLine is the plain line of trees' open phase when it opens
 // the database at once; it prints it as the phase starts and as it ends.
 const treesOpenLine = "[1/4] open: opening the database, elapsed 0s, " +
-	"eta unknown"
+	"remain unknown"
 
 // smokeTreeRecs mirrors the README smoke-test tree layout: /d/t1 and
 // /d/t2 are identical, /d/t3 differs from them only by one filename.
@@ -236,11 +236,11 @@ func TestRunTreesProgressNamesEachPhase(t *testing.T) {
 		"trees: database " + path + " (from " + databaseEnv + ")",
 		treesOpenLine,
 		treesOpenLine,
-		"[2/4] read: counting the records, elapsed 0s, eta unknown",
+		"[2/4] read: counting the records, elapsed 0s, remain unknown",
 		"[2/4] read: [6/6] 100%, ",
-		"[3/4] group: [0/8] 0%, 0 directories/s, elapsed 0s, eta unknown",
+		"[3/4] group: [0/8] 0%, 0 directories/s, elapsed 0s, remain unknown",
 		"[3/4] group: [8/8] 100%, ",
-		"[4/4] write: [0/1] 0%, 0 groups/s, elapsed 0s, eta unknown",
+		"[4/4] write: [0/1] 0%, 0 groups/s, elapsed 0s, remain unknown",
 		"[4/4] write: [1/1] 100%, ",
 		"trees: 6 records read, 1 duplicate tree groups, 1 dupe trees, ",
 	}