Bitmap generation: Add a test of ordering commits by "chains"

Bitmap generation: Add a test of ordering commits by "chains"

When commits are selected for bitmap generation, they are reordered
so that related "chains" of commits are grouped together. Chains are
"subbranches" of commits that may branch off of and re-merge with the
main line. Grouping by chains means that the XOR difference between
consecutive selected commits will be smaller, resulting in better
run-length compression of the XORed bitmaps.

Add a new testSelectionOrderingWithChains() test in a new
GcCommitSelectionTest test class. Also move related GC commit selection
tests out of GcBasicPackingTest and into GcCommitSelectionTest.

Change-Id: I8e80cac29c4ca8193b41c9898e5436c22a659f11
Signed-off-by: Terry Parker <tparker@google.com>