git-fix-whitespace series 1: Knowing about `git diff -p`

In the last post, the first requirement of the project (https://github.com/mrkschan/git-fix-whitespace) has been settled. The next requirement of this project is to read the git-diff patch in order to find any line changes that violate the whitespace rules specified in git config.

In a typical git-diff patch (see below), there are a few major parts.

1. Line 1 provides metadata about the modified file. It gives the file path to the file, rooted at the git repository.
2. Line 2 provides metadata about the git index and the file object discretionary access control list.
3. Line 3 and 4 provides metadata about which file path is old and which is new.
4. Line 5, 14, and 24 are metadata that tells which part of the file is modified. Let's take an example to explain this - "@@ -33,8 +33,7 @@ def sanitize_diff(git_diff):". "-33,8" tells that there is a diff hunk with 8 lines starts at line 33 of the old version of the file. "+33,7" tells that there is a diff hunk with 7 lines starts at line 33 of the new version of the file. As a result, the new version of the file gets one line fewer than the old version.
5. The rest of the patch is the content in the modified file. Those lines are either prefixed by ' ', '-', or '+'. ' ' means no modification, '-' means removed line, and '+' means added line. Note, there is no line replacement since it is represented by '-' lines followed by '+' lines (see line 28-31).

After knowing the structure of a git-diff patch, the next step is to read and write the modified file.