Merge branch 'pb/complete-commit-trailers' into next

The command-line complation support (in contrib/) learned to
complete "git commit --trailer=" for possible trailer keys.

* pb/complete-commit-trailers:
  completion: commit: complete configured trailer tokens

contrib/completion/git-completion.bash

@@ -1677,6 +1677,11 @@ _git_clone ()
 
 __git_untracked_file_modes="all no normal"
 
+__git_trailer_tokens ()
+{
+	git config --name-only --get-regexp trailer.\*.key | awk -F. '{print $2}'
+}
+
 _git_commit ()
 {
 	case "$prev" in
@@ -1701,6 +1706,10 @@ _git_commit ()
 		__gitcomp "$__git_untracked_file_modes" "" "${cur##--untracked-files=}"
 		return
 		;;
+	--trailer=*)
+		__gitcomp_nl "$(__git_trailer_tokens)" "" "${cur##--trailer=}" ":"
+		return
+		;;
 	--*)
 		__gitcomp_builtin commit
 		return