git-changelog: add --start-commit option, like --start-tag but for commit

[spacewander]

For #719

[spacewander]

@thisisshi
The --start-commit option is mostly like the --start-tag option, so it could be with other options.
For example, it could work with --list and --merges-only option:

￥ git changelog -x --start-commit @~3 -p -l -m
  * Merge pull request #721 from spacewander/git_changelog_list_newline
    git-changelog: add missing newline to the plain output
  * Merge pull request #713 from pabs3/docs
    Updates to the documentation building workflow and docs
  * Merge pull request #717 from flatcap/ignore-private
    add: git-ignore -p (private to repo)
  * Merge pull request #716 from francoism90/patch-1
    Add installation guide for Arch Linux

Note that the start commit is included in the changelog, as start tag is included in the changelog too.

[spacewander]

Merged.