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