Merge branch 'nd/maint-branch-desc-doc'

Merge branch 'nd/maint-branch-desc-doc'

Teach various forms of "format-patch" command line to identify what
branch the patches are taken from, so that the branch description
is picked up in more cases.

* nd/maint-branch-desc-doc:
  format-patch: pick up branch description when no ref is specified
  format-patch: pick up correct branch name from symbolic ref
  t4014: a few more tests on cover letter using branch description
  branch: delete branch description if it's empty
  config.txt: a few lines about branch.<name>.description