completion: restore removed line continuating backslash

Recent commit 1cd23e9e0 (completion: don't use __gitdir() for git
commands, 2017-02-03) rewrapped a couple of long lines, and while
doing so it inadvertently removed a '\' from the end of a line, thus
breaking completion for 'git config remote.name.push <TAB>'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2010,7 +2010,7 @@ _git_config ()
 	remote.*.push)
 		local remote="${prev#remote.}"
 		remote="${remote%.push}"
-		__gitcomp_nl "$(__git for-each-ref
+		__gitcomp_nl "$(__git for-each-ref \
 			--format='%(refname):%(refname)' refs/heads)"
 		return
 		;;