Merge branch 'jk/ui-color-always-to-auto'

Merge branch 'jk/ui-color-always-to-auto'

Fix regression of "git add -p" for users with "color.ui = always"
in their configuration, by merging the topic below and adjusting it
for the 'master' front.

* jk/ui-color-always-to-auto:
  t7301: use test_terminal to check color
  t4015: use --color with --color-moved
  color: make "always" the same as "auto" in config
  provide --color option for all ref-filter users
  t3205: use --color instead of color.branch=always
  t3203: drop "always" color test
  t6006: drop "always" color config tests
  t7502: use diff.noprefix for --verbose test
  t7508: use test_terminal for color output
  t3701: use test-terminal to collect color output
  t4015: prefer --color to -c color.diff=always
  test-terminal: set TERM=vt100