branch.c: replace `git_config()` with `git_config_get_string()

branch.c: replace `git_config()` with `git_config_get_string()

Use `git_config_get_string()` instead of `git_config()` to take advantage of
the config-set API which provides a cleaner control flow. While we are at
it, return -1 if we find no value for the queried variable. Original code
returned 0 for all cases, which was checked by `add_branch_desc()` in
fmt-merge-msg.c resulting in addition of a spurious newline to the `out`
strbuf. Now, the newline addition is skipped as -1 is returned to the caller
if no value is found.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>