When a .gitignore file contains the special rule to ignore every
subfolder content (`*/*`) and the file is located in a subfolder
relative to where the command is executed (root), the rule is
incorrectly applied and ignores every file at the same level of the
.gitignore file.

The reason for this is that the `gitignore` variable accumulates the
rules found in each .gitignore while traversing files and directories
recursively. This makes sense and, in general, works as expected. The
problem is that the gitignore rules are applied using as the relative
path from root to target directory as a reference. This is the cause
of the bug.

The implemented solution keeps track of every .gitignore file found
while traversing the targets and the absolute location of each
.gitignore file. Then, when matching files to the .gitignore rules,
compare each set of rules with the appropiate relative path to the
candidate target file.

To make this possible, we changed the single `gitignore` object with a
dictionary of similar objects, where the corresponding key is the
absolute path to the folder that contains that .gitignore file. This
required changing the signature of the `get_sources` function. Also, we
introduce a `is_ignored` function that compares a file with every set
of rules. Finally, some tests required an update to pass the gitignore
object in the new format.

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

The test contains three cases: 1) when the .gitignore with the special
rule to ignore every subfolder and its contents (*/*) is in the root,
2) when the file is inside a subfolder relative to root (nested), and
3) when the target folder contains the .gitignore and root is a parent
folder of the target. In all of these cases, we compare the files that
are visible by Black with a known list of paths containing the
expected values.

Before the fix introduced in the previous commit, these tests failed
when the .gitignore file was nested (second case). Now, the test is
passed for all cases.

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

Add entry about fixed bug and changes introduced: ignore files by
considering the location of each .gitignore file and the relative path
of each target

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

These changes are small improvements to improve code readability:
rename a variable to a more descriptive name (from `exclude_is_None`
to `using_default_exclude`), use a better syntax to include the type
annotation for `gitignore` variable (from typing comment to
Python-style typing annotation), and replace an if-else block with a
single dictionary definition (in this case, we need to compare keys
instead of values, meaning that the change works)

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

The function to match a given path with every discovered .gitignore
file does not need to be a nested function and can be a top-level
function. The arguments did not change, but the naming of local
variables was improved for readability.

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