Merge branch 'ds/maintenance-part-1'

A "git gc"'s big brother has been introduced to take care of more
repository maintenance tasks, not limited to the object database
cleaning.

* ds/maintenance-part-1:
  maintenance: add trace2 regions for task execution
  maintenance: add auto condition for commit-graph task
  maintenance: use pointers to check --auto
  maintenance: create maintenance.<task>.enabled config
  maintenance: take a lock on the objects directory
  maintenance: add --task option
  maintenance: add commit-graph task
  maintenance: initialize task array
  maintenance: replace run_auto_gc()
  maintenance: add --quiet option
  maintenance: create basic maintenance runner

diff --cc .gitignore
Simple merge

diff --cc Documentation/fetch-options.txt
Simple merge

diff --cc builtin.h
Simple merge

diff --cc builtin/am.c
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/merge.c
Simple merge

diff --cc builtin/rebase.c
Simple merge

diff --cc commit-graph.c
Simple merge

diff --cc git.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge