clang-format: avoid spacing around bitfield colon

clang-format: avoid spacing around bitfield colon

The spacing around colons is currently not standardized and as such we
have the following practices in our code base:
- Spacing around the colon `int bf : 1`: 146 instances
- No spacing around the colon `int bf:1`: 148 instances
- Spacing before the colon `int bf :1`: 6 instances
- Spacing after the colon `int bf: 1`: 12 instances

Let's formalize this by picking the most followed pattern and add the
corresponding style to '.clang-format'.

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