Add logging for bases in incremental backups (#2151)

## Description

Add log statements noting which bases were used for kopia assisted incrementals and which bases were merged into the hierarchy. Also record the reasons a base was chosen.

Log statements when searching for previous snapshots will be added when that code is refactored

## Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No 

## Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

## Issue(s)

* #2149 

## Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/upload.go

@@ -15,6 +15,7 @@ import (
 	"github.com/hashicorp/go-multierror"
 	"github.com/kopia/kopia/fs"
 	"github.com/kopia/kopia/fs/virtualfs"
+	"github.com/kopia/kopia/repo/manifest"
 	"github.com/kopia/kopia/snapshot/snapshotfs"
 	"github.com/pkg/errors"
 
@@ -884,6 +885,17 @@ func inflateDirTree(
 		return nil, errors.Wrap(err, "inflating collection tree")
 	}
 
+	baseIDs := make([]manifest.ID, 0, len(baseSnaps))
+	for _, snap := range baseSnaps {
+		baseIDs = append(baseIDs, snap.ID)
+	}
+
+	logger.Ctx(ctx).Infow(
+		"merging hierarchies from base snapshots",
+		"snapshot_ids",
+		baseIDs,
+	)
+
 	for _, snap := range baseSnaps {
 		if err = inflateBaseTree(ctx, loader, snap, updatedPaths, roots); err != nil {
 			return nil, errors.Wrap(err, "inflating base snapshot tree(s)")

src/internal/kopia/wrapper.go

@@ -178,11 +178,20 @@ func (w Wrapper) makeSnapshotWithRoot(
 		bc  = &stats.ByteCounter{}
 	)
 
+	snapIDs := make([]manifest.ID, 0, len(prevSnapEntries))
 	prevSnaps := make([]*snapshot.Manifest, 0, len(prevSnapEntries))
+
 	for _, ent := range prevSnapEntries {
 		prevSnaps = append(prevSnaps, ent.Manifest)
+		snapIDs = append(snapIDs, ent.ID)
 	}
 
+	logger.Ctx(ctx).Infow(
+		"using snapshots for kopia-assisted incrementals",
+		"snapshot_ids",
+		snapIDs,
+	)
+
 	err := repo.WriteSession(
 		ctx,
 		w.c,

src/internal/operations/backup.go

@@ -340,6 +340,8 @@ func consumeBackupDataCollections(
 
 	for _, m := range mans {
 		paths := make([]*path.Builder, 0, len(m.Reasons))
+		services := map[string]struct{}{}
+		categories := map[string]struct{}{}
 
 		for _, reason := range m.Reasons {
 			pb, err := builderFromReason(tenantID, reason)
@@ -348,12 +350,34 @@ func consumeBackupDataCollections(
 			}
 
 			paths = append(paths, pb)
+			services[reason.Service.String()] = struct{}{}
+			categories[reason.Category.String()] = struct{}{}
 		}
 
 		bases = append(bases, kopia.IncrementalBase{
 			Manifest:     m.Manifest,
 			SubtreePaths: paths,
 		})
+
+		svcs := make([]string, 0, len(services))
+		for k := range services {
+			svcs = append(svcs, k)
+		}
+
+		cats := make([]string, 0, len(categories))
+		for k := range categories {
+			cats = append(cats, k)
+		}
+
+		logger.Ctx(ctx).Infow(
+			"using base for backup",
+			"snapshot_id",
+			m.ID,
+			"services",
+			svcs,
+			"categories",
+			cats,
+		)
 	}
 
 	return bu.BackupCollections(ctx, bases, cs, tags, isIncremental)