test: mock git for some tests to improve code coverage

[nexdrew]

Mocks git (for a subset of tests) in order to better test:

• --sign attempts to sign the commit and tag
• exits with error if git commit fails
• exits with error if git tag fails

This improves code/line coverage from 91.57 to 95.18 percent.

Still need a way to induce/mock error scenarios for conventionalRecommendedBump and conventionalChangelog.

[coveralls]

Coverage increased (+3.6%) to 95.181% when pulling b919f56 on nexdrew:better-sign-test into b6b2a75 on conventional-changelog:master.

[stevemao]

Wow this is interesting :) Can definitely be refactored into a module :)

[nexdrew]

@stevemao Hmm, not a bad idea, thanks!