Extends git_hook to allow for a user-specified configuration file.

The behavior of the `git_hook` function was to search for a configuration file
by starting from the directory containing the first staged file and moving
upwards until a configuration file was found or a certain number of directories
were checked.

If the project configuration file is not stored in a directory that is a parent
to each python file in the codebase, the `git_hook` function was unable to
retrieve the file.

For example, this might happen with a configuration such as the following:

    ```
    git-root/
      config/
        .isort.cfg
      src/
        ...
    ```

To allow for the `git_hook` function to support those project structures without
breaking backwards compatibility, an optional argument, `settings_file`, was
added to it.

When `settings_file` is the empty string the function behaves as before.
Otherwise, `settings_file` is considered a path to a valid configuration file
that will be used for the hook's run.

A test in `tests/test_hooks.py` was added to cover for this case.

`README.md#Git hook` was modified to mention the new interface.

1 files changed, 10 insertions, 1 deletions

diff --git a/README.md b/README.md
index 99e91147..a77e2017 100644
--- a/README.md
+++ b/README.md
@@ -556,7 +556,7 @@ include the following in `.git/hooks/pre-commit`:
 import sys
 from isort.hooks import git_hook
 
-sys.exit(git_hook(strict=True, modify=True, lazy=True))
+sys.exit(git_hook(strict=True, modify=True, lazy=True, settings_file=""))
 ```
 
 If you just want to display warnings, but allow the commit to happen
@@ -569,6 +569,15 @@ Set it to `True` to ensure all tracked files are properly isorted,
 leave it out or set it to `False` to check only files added to your
 index.
 
+If you want to use a specific configuration file for the hook, you can pass its
+path to settings_file. If no path is specifically requested, `git_hook` will
+search for the configuration file starting at the directory containing the first
+staged file, as per `git diff-index` ordering, and going upward in the directory
+structure until a valid configuration file is found or
+[`MAX_CONFIG_SEARCH_DEPTH`](src/config.py:35) directories are checked.
+The settings_file parameter is used to support users who keep their configuration
+file in a directory that might not be a parent of all the other files.
+
 ## Setuptools integration
 
 Upon installation, isort enables a `setuptools` command that checks