config: rename git_config_set to git_config_set_gently

config: rename git_config_set to git_config_set_gently

The desired default behavior for `git_config_set` is to die
whenever an error occurs. Dying is the default for a lot of
internal functions when failures occur and is in this case the
right thing to do for most callers as otherwise we might run into
inconsistent repositories without noticing.

As some code may rely on the actual return values for
`git_config_set` we still require the ability to invoke these
functions without aborting. Rename the existing `git_config_set`
functions to `git_config_set_gently` to keep them available for
those callers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>