Add verification in GcKeepFilesTest that bitmaps are generated

The packfiles with the .keep extensions are meant to prevent
a packfile from being processed or removed during GC.
From the point of view of the GC process then, the associated
packfile should be completely transparent:
- it should not included in the repacked file
- it should not pruned
- its objects should be left untouched, even if unreachable
- the GC process, including the bitmap generation should continue
  as usual, as the the packfiles with .keep file did not exist

Add one explicit test for making sure that the management
of .keep file is also transparent to the generation of bitmaps,
which are still generated if a .keep file exists.

Bug: 582039
Change-Id: I14f6adc3f961c606fbc617e51ea6ed6e2ef8604f

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcKeepFilesTest.java

@@ -48,11 +48,15 @@ public void testKeepFiles() throws Exception {
 		assertEquals(4, stats.numberOfLooseObjects);
 		assertEquals(4, stats.numberOfPackedObjects);
 		assertEquals(1, stats.numberOfPackFiles);
+		PackFile bitmapFile = singlePack.getPackFile().create(PackExt.BITMAP_INDEX);
+		assertTrue(keepFile.exists());
+		assertTrue(bitmapFile.delete());
 		gc.gc();
 		stats = gc.getStatistics();
 		assertEquals(0, stats.numberOfLooseObjects);
 		assertEquals(8, stats.numberOfPackedObjects);
 		assertEquals(2, stats.numberOfPackFiles);
+		assertEquals(1, stats.numberOfBitmaps);
 
 		// check that no object is packed twice
 		Iterator<Pack> packs = repo.getObjectDatabase().getPacks()