command-bar: Fix vim mode tab autocompletion issues

gb_vim_complete_edit_files returns the completed file path as an
absolute path even when the user typed a relative path. This commit
makes the completion look like the user input even for relative paths,
which allows gb_command_bar_complete to correctly complete file paths
for the user.

https://bugzilla.gnome.org/show_bug.cgi?id=767369