PackFile: Cache the packName string

Instead of computing this on every request, compute it once and
hold onto the result. This improves performance for LocalCachedPack
which does a lot of tests against the pack name string.

Change-Id: I3803745e3a5dda7b5f0faf39aae9423e2c777e7f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/PackFile.java

@@ -96,6 +96,8 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 
 	private final File packFile;
 
+	private volatile String packName;
+
 	final int hash;
 
 	private RandomAccessFile fd;
@@ -177,11 +179,15 @@ public class PackFile implements Iterable<PackIndex.MutableEntry> {
 
 	/** @return name extracted from {@code pack-*.pack} pattern. */
 	public String getPackName() {
-		String name = getPackFile().getName();
-		if (name.startsWith("pack-"))
-			name = name.substring("pack-".length());
-		if (name.endsWith(".pack"))
-			name = name.substring(0, name.length() - ".pack".length());
+		String name = packName;
+		if (name == null) {
+			name = getPackFile().getName();
+			if (name.startsWith("pack-"))
+				name = name.substring("pack-".length());
+			if (name.endsWith(".pack"))
+				name = name.substring(0, name.length() - ".pack".length());
+			packName = name;
+		}
 		return name;
 	}