completion: add space after config variable names also in Bash 3

In be6444d (completion: bash: add correct suffix in variables,
2021-08-16), __git_complete_config_variable_name was changed to use
"${sfx- }" instead of "$sfx" as the fourth argument of _gitcomp_nl and
_gitcomp_nl_append, such that this argument evaluates to a space if sfx
is unset. This was to ensure that e.g.

	git config branch.autoSetupMe[TAB]

correctly completes to 'branch.autoSetupMerge ' with the trailing space.
This commits notes that the fix only works in Bash 4 because in Bash 3
the 'local sfx' construct at the beginning of
__git_complete_config_variable_name creates an empty string.

Make the fix also work for Bash 3 by using the "unset or null' parameter
expansion syntax ("${sfx:- }"), such that the parameter is also expanded
to a space if it is set but null, as is the behaviour of 'local sfx' in
Bash 3.

Signed-off-by: Philippe Blain <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phil-blain

contrib/completion/git-completion.bash

@@ -2750,7 +2750,7 @@ __git_complete_config_variable_name ()
 		local pfx="${cur_%.*}."
 		cur_="${cur_#*.}"
 		__gitcomp_direct "$(__git_heads "$pfx" "$cur_" ".")"
-		__gitcomp_nl_append $'autoSetupMerge\nautoSetupRebase\n' "$pfx" "$cur_" "${sfx- }"
+		__gitcomp_nl_append $'autoSetupMerge\nautoSetupRebase\n' "$pfx" "$cur_" "${sfx:- }"
 		return
 		;;
 	guitool.*.*)
@@ -2784,7 +2784,7 @@ __git_complete_config_variable_name ()
 		local pfx="${cur_%.*}."
 		cur_="${cur_#*.}"
 		__git_compute_all_commands
-		__gitcomp_nl "$__git_all_commands" "$pfx" "$cur_" "${sfx- }"
+		__gitcomp_nl "$__git_all_commands" "$pfx" "$cur_" "${sfx:- }"
 		return
 		;;
 	remote.*.*)
@@ -2800,7 +2800,7 @@ __git_complete_config_variable_name ()
 		local pfx="${cur_%.*}."
 		cur_="${cur_#*.}"
 		__gitcomp_nl "$(__git_remotes)" "$pfx" "$cur_" "."
-		__gitcomp_nl_append "pushDefault" "$pfx" "$cur_" "${sfx- }"
+		__gitcomp_nl_append "pushDefault" "$pfx" "$cur_" "${sfx:- }"
 		return
 		;;
 	url.*.*)