completion: simplify prefix path component handling during path completion

Once upon a time 'git -C "" cmd' errored out with "Cannot change to
'': No such file or directory", therefore the completion script took
extra steps to run 'git -C "." cmd' instead; see fca416a41e
(completion: use "git -C $there" instead of (cd $there && git ...),
2014-10-09).

Those extra steps are not needed since 6a536e2076 (git: treat "git -C
'<path>'" as a no-op when <path> is empty, 2015-03-06), so remove
them.

While at it, also simplify how the trailing '/' is appended to the
variable holding the prefix path components.

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

contrib/completion/git-completion.bash

@@ -387,7 +387,7 @@ __git_ls_files_helper ()
 #    slash.
 __git_index_files ()
 {
-	local root="${2-.}" file
+	local root="$2" file
 
 	__git_ls_files_helper "$root" "$1" |
 	while read -r file; do
@@ -408,13 +408,12 @@ __git_complete_index_file ()
 
 	case "$cur_" in
 	?*/*)
-		pfx="${cur_%/*}"
+		pfx="${cur_%/*}/"
 		cur_="${cur_##*/}"
-		pfx="${pfx}/"
 		;;
 	esac
 
-	__gitcomp_file "$(__git_index_files "$1" ${pfx:+"$pfx"})" "$pfx" "$cur_"
+	__gitcomp_file "$(__git_index_files "$1" "$pfx")" "$pfx" "$cur_"
 }
 
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