`gitconfig-indent-line': compare `char-after' with `[' using `equal'

Calling `newline-and-indent' with point being @ `point-max' resulted in
a `wrong-type-argument' error due to `gitconfig-indent-line' trying to
compare `char-after's return value (which is nil @ eob) to `[' using
`=', which expects both of its arguments to be numbers or markers.

Signed-off-by: Pieter Praet <pieter@praet.org>