config: create a function to format section headers

config: create a function to format section headers

Factor out the logic which creates section headers in the config file,
e.g. the 'branch.foo' key will be turned into '[branch "foo"]'.

This introduces no function changes, but is needed for a later change
which adds support for copying branch sections in the config file.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Ramsay Jones <redacted>
Signed-off-by: Sahil Dua <redacted>
Signed-off-by: Junio C Hamano <redacted>