Adding support for git blame.

[suvamM]

This PR adds support for enriching a SARIF log with git blame information. The high level algorithm is as follows:

1. For each result, if the corresponding artifact is a file, retrieve blame information associated with the file.
2. Parse the blame information to extract relevant information (such as author name, email, etc).
3. If the code location of the SARIF result overlaps with the blame hunk, enrich the result node with the extracted blame information.

The code changes have been manually tested, but the PR currently does not introduce unit tests for the feature. The problem lies with mocking appropriate external services, since the relevant code creates local objects to interact with these external services (such as GitHelper).

Also, the OptionallyEmittedData.All flag now assumes slightly different semantics, where it implies all flags except the overwrite and git blame flags. This was necessary since the existing test for the OptionallyEmittedData.All flag does not support the preconditions for git blame.

[lgtm-com]

This pull request introduces 1 alert when merging c2cb3f3 into 2f3eb01 - view on LGTM.com

new alerts:

• 1 for Dereferenced variable may be null

[suvamM]

@eddynaka Made a commit incorporating your suggestions. The unit test for the git blame parser uncovered a couple of bugs! Things are fixed now. Look forward to your feedback.