clang-format: indent preprocessor directives after hash

clang-format: indent preprocessor directives after hash

We do not have a rule around the indentation of preprocessor directives.
This was also discussed on the list [1], noting how there is often
inconsistency in the styling. While there was discussion, there was no
conclusion around what is the preferred style here. One style being
indenting after the hash:

    #if FOO
    #  if BAR
    #    include <foo>
    #  endif
    #endif

The other being before the hash:

    #if FOO
      #if BAR
        #include <foo>
      #endif
    #endif

Let's pick the former and add 'IndentPPDirectives: AfterHash' value to
our '.clang-format'. There is no clear reason to pick one over the
other, but it would definitely be nicer to be consistent.

[1]: https://lore.kernel.org/r/xmqqwmmm1bw6.fsf@gitster.g

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>