config: add '--name-only' option to list only variable names

'git config' can only show values or name-value pairs, so if a shell
script needs the names of set config variables it has to run 'git config
--list' or '--get-regexp' and parse the output to separate config
variable names from their values.  However, such a parsing can't cope
with multi-line values.  Though 'git config' can produce null-terminated
output for newline-safe parsing, that's of no use in such a case, becase
shells can't cope with null characters.

Even our own bash completion script suffers from these issues.

Help the completion script, and shell scripts in general, by introducing
the '--name-only' option to modify the output of '--list' and
'--get-regexp' to list only the names of config variables, so they don't
have to perform error-prone post processing to separate variable names
from their values anymore.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1887,6 +1887,7 @@ _git_config ()
 			--get --get-all --get-regexp
 			--add --unset --unset-all
 			--remove-section --rename-section
+			--name-only
 			"
 		return
 		;;