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

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/log.c
Simple merge

diff --cc t/t4014-format-patch.sh
Simple merge