commit-graph: add new config for changed-paths & recommend it in scalar

The changed-path Bloom filters feature has proven stable and reliable
over several years of use, delivering significant performance
improvement for file history computation in large monorepos. Currently
a user can opt-in to writing the changed-path Bloom filters using the
"--changed-paths" option to "git commit-graph write". The filters will
be persisted until the user drops the filters using the
"--no-changed-paths" option. For this functionality, refer to 0087a87ba8
(commit-graph: persist existence of changed-paths, 2020-07-01).

Large monorepos using Git's background maintenance to build and update
commit-graph files could use an easy switch to enable this feature
without a foreground computation. In this commit, we're proposing a new
config option "commitGraph.changedPaths":

* If "true", "git commit-graph write" will write Bloom filters,
  equivalent to passing "--changed-paths";
* If "false" or "unset", Bloom filters will be written during "git
  commit-graph write" only if the filters already exist in the current
  commit-graph file. This matches the default behaviour of "git
  commit-graph write" without any "--[no-]changed-paths" option. Note
  "false" can disable a previous "true" config value but doesn't imply
  "--no-changed-paths".

This config will always respect the precedence of command line option
"--[no-]changed-paths".

We also set this new config as optional recommended config in scalar to
turn on this feature for large repos.

Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Emily Yang <emilyyang.git@gmail.com>
Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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