Apply .gitignore correctly in every source entry (#3336)

When passing multiple src directories, the root gitignore was only
applied to the first processed source. The reason is that, in the
first source, exclude is `None`, but then the value gets overridden by
`re_compile_maybe_verbose(DEFAULT_EXCLUDES)`, so in the next iteration
where the source is a directory, the condition is not met and sets the
value of `gitignore` to `None`.

To fix this problem, we store a boolean indicating if `exclude` is
`None` and set the value of `exclude` to its default value if that's
the case. This makes sure that the flow enters the correct condition on
following iterations and also keeps the original value if the condition
is not met.

Also, the value of `gitignore` is initialized as `None` and overriden
if necessary. The value of `root_gitignore` is always calculated to
avoid using additional variables (at the small cost of additional
computations).

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>