clang-format: fix indentation width for preprocessor directives

In [1], we have improved our clang-format configuration to also specify the style for how to indent preprocessor directives. But while we have settled the question of where to put the indentation, either before or after the hash sign, we didn't specify exactly how to indent. With the current configuration, clang-format uses tabs to indent each level of nested preprocessor directives, which is in fact unintentional and never done in our codebase. Instead, we use a mixture of indenting by either one or two spaces, where using a single space is somewhat more common. Adapt our clang-format configuration accordingly by specifying an indentation width of one space. [1]: <20240708092317.267915-1-karthik.188@gmail.com> Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>