Make **/ in an exclude pattern match zero directories (closes #338)

The glob library needs a slash on both sides of **, so the anchored
pattern /**/x missed an x directly under the snapshot path, and
a/**/*.log missed a/x.log. As in gitignore, a leading **/ now makes any
pattern, anchored or not, match at every depth, and a /**/ inside a
pattern also matches a single slash.

Judgement call: the test for a/**/b uses a/**/*.log, because with a
plain name after the ** the glob library already matched a/b.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -2023,13 +2023,20 @@ func compileExcludePatterns(patterns []string) []compiledPattern {
 		pattern = strings.TrimSuffix(pattern, "/")
 
 		// A leading "**/" demands a "/" before the name and so misses an
-		// entry directly under the source path. An unanchored pattern
-		// already matches at every depth, so drop the prefix: as in
-		// gitignore, "**/x" then also matches a top-level "x".
-		if !anchored {
+		// entry directly under the source path. It means "at every depth",
+		// which is what an unanchored pattern already does, so drop the
+		// prefix and match unanchored: as in gitignore, "**/x" and
+		// "/**/x" then also match a top-level "x".
+		if strings.HasPrefix(pattern, "**/") {
 			pattern = strings.TrimPrefix(pattern, "**/")
+			anchored = false
 		}
 
+		// Likewise "/**/" inside a pattern demands two slashes, so
+		// "a/**/b" misses "a/b". As in gitignore it stands for zero or
+		// more directories, so let it match a single "/" too.
+		pattern = strings.ReplaceAll(pattern, "/**/", "{/,/**/}")
+
 		// Compile the glob pattern
 		// For patterns without path separators, we need to match them as components
 		// e.g., ".git" should match ".git" anywhere in the path