CommitGraphWriter: add option for writing/using bloom filters

CommitGraphWriter: add option for writing/using bloom filters

Currently, bloom filters are written and used without any way to turn
them off. Add a per-repo config variable to control whether bloom
filters are written. As for reading, add a JGit option to control this.
(A JGit option is used instead of a per-repo config variable as there is
usually no reason not to use the bloom filters if they are present, but
a global control to disable them is useful if there turns out to be an
issue with the implementation of bloom filters.)

The config that controls reading is the same as C Git, but the config
for writing is not: C Git has no config to control writing, but whether
bloom filters are written depends on whether bloom filters are already
present and what arguments are passed to "git commit-graph write". See
the manpage of "git commit-graph" for more information.

Change-Id: I1b7b25340387673506252b9260b22bfe147bde58