Merge branch 'rr/completion-format-coverletter'

The bash/zsh completion code did not know about format.coverLetter
among many format.* configuration variables.

* rr/completion-format-coverletter:
  completion: complete format.coverLetter

contrib/completion/git-completion.bash

@@ -2007,6 +2007,7 @@ _git_config ()
 		fetch.unpackLimit
 		format.attach
 		format.cc
+		format.coverLetter
 		format.headers
 		format.numbered
 		format.pretty