Docs: add GitCatFile and GitShow wrapper classes to the DG (#1590)

The git wrapper classes `GitShow` and `GitCatFile` were introduced in
PR #1449. However, the corresponding documentations were not included
in the Developer Guide.

Adding the wrapper classes link and description can help newcomers to
learn about the implementation design.

Let's add the documentation for these git wrapper classes into the
Developer Guide.

docs/dg/architecture.md

@@ -29,6 +29,7 @@
 [`Git`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git) package contains the wrapper classes for respective *git* commands.
  * [`GitBlame`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitBlame.java): Wrapper class for `git blame` functionality. Traces the revision and author last modified each line of a file.
  * [`GitBranch`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitBranch.java): Wrapper class for `git branch` functionality. Gets the name of the working branch of the target repo.
+ * [`GitCatFile`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitCatFile.java): Wrapper class for `git cat-file` functionality. Obtains the parent commit hash with the given commit indicated by the commit hash.
  * [`GitCheckout`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitCheckout.java): Wrapper class for `git checkout` functionality. Checks out the repository by branch name or commit hash.
  * [`GitClone`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitClone.java): Wrapper class for `git clone` functionality. Clones the repository from *GitHub* into a temporary folder in order to run the analysis.
  * [`GitDiff`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitDiff.java): Wrapper class for `git diff` functionality. Obtains the changes between commits.
@@ -37,6 +38,7 @@
  * [`GitRevList`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitRevList.java): Wrapper class for `git rev-list` functionality. Retrieves the commit objects in reverse chronological order.
  * [`GitRevParse`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitRevParse.java): Wrapper class for `git rev-parse` functionality. Ensures that the branch of the repo is to be analyzed exists.
  * [`GitShortlog`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitShortlog.java): Wrapper class for `git shortlog` functionality. Obtains the list of authors who have contributed to the target repo.
+ * [`GitShow`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitShow.java): Wrapper class for `git show` functionality. Gets the date of the commit with the commit hash.
  * [`GitUtil`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitUtil.java): Contains helper functions used by the other Git classes above.
  * [`GitVersion`](https://github.com/reposense/RepoSense/blob/master/src/main/java/reposense/git/GitVersion.java): Wrapper class for `git --version` functionality. Obtains the current git version of the environment that RepoSense is being run on.