submodule: factor out a config_set_in_gitmodules_file_gently function
Introduce a new config_set_in_gitmodules_file_gently() function to write
config values to the .gitmodules file.
This is in preparation for a future change which will use the function
to write to the .gitmodules file in a more controlled way instead of
using "git config -f .gitmodules".
The purpose of the change is mainly to centralize the code that writes
to the .gitmodules file to avoid some duplication.
The naming follows git_config_set_in_file_gently() but the git_ prefix
is removed to communicate that this is not a generic git-config API.
Signed-off-by: Antonio Ospite <ao2@ao2.it> Signed-off-by: Junio C Hamano <gitster@pobox.com>