Merge branch 'ps/maintenance-geometric'

"git maintenance" command learns the "geometric" strategy where it
avoids doing maintenance tasks that rebuilds everything from
scratch.

* ps/maintenance-geometric:
  t7900: fix a flaky test due to git-repack always regenerating MIDX
  builtin/maintenance: introduce "geometric" strategy
  builtin/maintenance: make "gc" strategy accessible
  builtin/maintenance: extend "maintenance.strategy" to manual maintenance
  builtin/maintenance: run maintenance tasks depending on type
  builtin/maintenance: improve readability of strategies
  builtin/maintenance: don't silently ignore invalid strategy
  builtin/maintenance: make the geometric factor configurable
  builtin/maintenance: introduce "geometric-repack" task
  builtin/gc: make `too_many_loose_objects()` reusable without GC config
  builtin/gc: remove global `repack` variable

Author: gitster

Documentation/config/maintenance.adoc

@@ -16,19 +16,36 @@ detach.
 
 maintenance.strategy::
 	This string config option provides a way to specify one of a few
-	recommended schedules for background maintenance. This only affects
-	which tasks are run during `git maintenance run --schedule=X`
-	commands, provided no `--task=<task>` arguments are provided.
-	Further, if a `maintenance.<task>.schedule` config value is set,
-	then that value is used instead of the one provided by
-	`maintenance.strategy`. The possible strategy strings are:
+	recommended strategies for repository maintenance. This affects
+	which tasks are run during `git maintenance run`, provided no
+	`--task=<task>` arguments are provided. This setting impacts manual
+	maintenance, auto-maintenance as well as scheduled maintenance. The
+	tasks that run may be different depending on the maintenance type.
 +
-* `none`: This default setting implies no tasks are run at any schedule.
+The maintenance strategy can be further tweaked by setting
+`maintenance.<task>.enabled` and `maintenance.<task>.schedule`. If set, these
+values are used instead of the defaults provided by `maintenance.strategy`.
++
+The possible strategies are:
++
+* `none`: This strategy implies no tasks are run at all. This is the default
+  strategy for scheduled maintenance.
+* `gc`: This strategy runs the `gc` task. This is the default strategy for
+  manual maintenance.
+* `geometric`: This strategy performs geometric repacking of packfiles and
+  keeps auxiliary data structures up-to-date. The strategy expires data in the
+  reflog and removes worktrees that cannot be located anymore. When the
+  geometric repacking strategy would decide to do an all-into-one repack, then
+  the strategy generates a cruft pack for all unreachable objects. Objects that
+  are already part of a cruft pack will be expired.
++
+This repacking strategy is a full replacement for the `gc` strategy and is
+recommended for large repositories.
 * `incremental`: This setting optimizes for performing small maintenance
   activities that do not delete any data. This does not schedule the `gc`
   task, but runs the `prefetch` and `commit-graph` tasks hourly, the
   `loose-objects` and `incremental-repack` tasks daily, and the `pack-refs`
-  task weekly.
+  task weekly. Manual repository maintenance uses the `gc` task.
 
 maintenance.<task>.enabled::
 	This boolean config option controls whether the maintenance task
@@ -75,6 +92,22 @@ maintenance.incremental-repack.auto::
 	number of pack-files not in the multi-pack-index is at least the value
 	of `maintenance.incremental-repack.auto`. The default value is 10.
 
+maintenance.geometric-repack.auto::
+	This integer config option controls how often the `geometric-repack`
+	task should be run as part of `git maintenance run --auto`. If zero,
+	then the `geometric-repack` task will not run with the `--auto`
+	option. A negative value will force the task to run every time.
+	Otherwise, a positive value implies the command should run either when
+	there are packfiles that need to be merged together to retain the
+	geometric progression, or when there are at least this many loose
+	objects that would be written into a new packfile. The default value is
+	100.
+
+maintenance.geometric-repack.splitFactor::
+	This integer config option controls the factor used for the geometric
+	sequence. See the `--geometric=` option in linkgit:git-repack[1] for
+	more details. Defaults to `2`.
+
 maintenance.reflog-expire.auto::
 	This integer config option controls how often the `reflog-expire` task
 	should be run as part of `git maintenance run --auto`. If zero, then