Add a boolean `sync.smartsync` setting in the per-user repo config
(~/.repoconfig/config):
* true: smart sync by default, as if -s were passed.
* false: never smart sync by default, even if the manifest enables it.
* unset: follow the manifest default, as before.
Add docs/repoconfig.md as the user facing documentation for
~/.repoconfig/config and its settings, and point smart-sync.md and the
`repo sync` help at it.
Bug: 569927750
Test: ./run_tests
Change-Id: Ideb4119fcb5e90f8bb35c493d9e4f8bec7878b77
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/643264
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Insaf Latypov <saferif@google.com>
Tested-by: Insaf Latypov <saferif@google.com>
