feat(core): source control commands for retrieving commits

These two commands will be used to retrieve commits from the source
control provider. One retrieves the latest, current commit; and the
other returns a list of commits. The commit list can accept from and to
arguments that filter the list of commits that will be returned.

Ref: #75

src/lib/source-control/git.ts

@@ -1,4 +1,5 @@
 import {run} from '../exec';
+import * as fs from 'fs';
 import type {SourceControlProvider} from '.';
 
 const git: SourceControlProvider = {
@@ -17,6 +18,37 @@ const git: SourceControlProvider = {
     const {stdout} = await run(`git rev-parse --show-toplevel`);
     return stdout.trim();
   },
+  getCurrentCommitMessage: async () => {
+    const root = await git.root();
+
+    const commitEditMessagePath = `${root}/.git/COMMIT_EDITMSG`;
+
+    if (fs.existsSync(commitEditMessagePath)) {
+        const commitEditMessage = fs
+            .readFileSync(commitEditMessagePath)
+            .toString();
+
+        const scissors = commitEditMessage.indexOf(
+            '------------------------ >8 ------------------------',
+        );
+
+        const end = scissors > -1 ? scissors : commitEditMessage.length;
+        const editMessage = commitEditMessage.substring(0, end).replace(/^#.*$/gm, '').trim();
+
+        if (editMessage.length) {
+            return editMessage;
+        }
+    }
+
+    return null;
+  },
+  getCommits: async (from, to) => {
+    const range = from && to ? `${from}..${to}` : '';
+    const {stdout: commitsString} = await run(
+      `git log ${range} --pretty=format:"%B%x00"`,
+    );
+    return commitsString.split('\0');
+  },
 };
 
 export default git;

src/lib/source-control/index.ts

@@ -16,6 +16,19 @@ export interface SourceControlProvider {
    * Get the current root directory of for the repository.
    */
   root(): Promise<string>;
+  /**
+   * Get the current commit message from the source provider.
+   */
+  getCurrentCommitMessage(): Promise<string | null>;
+  /**
+   * Get a list of commits from the source provider to lint.
+   *
+   * @param from A string representing the starting commit hash or reference to
+   * begin retrieving commits from.
+   * @param to A string representing the ending commit hash or reference to
+   * stop retrieving commits from.
+   */
+  getCommits(from?: string, to?: string): Promise<string[]>;
 }
 
 const PROVIDERS = [git, sapling];