clang-format: fix indentation width for preprocessor directives

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>