sequencer: improve config handling

sequencer: improve config handling

The previous config handling relied on global variables, called
git_default_config() even when the key had already been handled by
git_sequencer_config() and did not initialize the diff configuration
variables. Improve this by: i) loading the default values for message
cleanup and gpg signing of commits into struct replay_opts;
ii) restructuring the code to return immediately once a key is
handled; and iii) calling git_diff_basic_config(). Note that
unfortunately it is not possible to return early if the key is handled
by git_gpg_config() as it does not indicate to the caller if the key
has been handled or not.

The sequencer should probably have been calling
git_diff_basic_config() before as it creates a patch when there are
conflicts. The shell version uses 'diff-tree' to create the patch so
calling git_diff_basic_config() should match that. Although 'git
commit' calls git_diff_ui_config() I don't think the output of
print_commit_summary() is affected by anything that is loaded by that
as print_commit_summary() always turns on rename detection so would
ignore the value in the user's configuration anyway. The other values
loaded by git_diff_ui_config() are about the formatting of patches so
are not relevant to print_commit_summary().

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>