GC: Trim EWAHCompressedBitmaps to free unused memory

GC: Trim EWAHCompressedBitmaps to free unused memory

The "Building bitmaps" GC phase fails for large repositories (repos with
10M objects use 1.25MB per uncompressed bitmap, and those with long
histories may build >25k bitmaps). Since these bitmaps xor well against
each other, the actual space needed for each compressed bitmap is
usually no more than a few KB. Calling trim() will ensure we aren't
holding on to excess memory.

Change-Id: I40bf78c730b9f6051da6025f9777ce27220a5b0a
Signed-off-by: Terry Parker <tparker@google.com>