Convert get_modification_date_for to use Git object instead of file_modification_cache.

Signed-off-by: Frank Kotsianas <frankkotsianas@gmail.com>

Makes middleman runner handle GitExecuteErrors.

Adds tests for get_modified_date_for method in sections.
Removes file_modification_cache from bookbinder. [Finishes #82256712]

Removes file modification cache from Publisher.

Removes GitModCache from Publish command.

Removes GitModCache and GitFileWalker for modified dates.

Signed-off-by: Brenda Chan <brchan@pivotal.io>
Removes write_file_modification_dates_to from section.rb.

Signed-off-by: Frank Kotsianas <frankkotsianas@gmail.com>
Removes DocRepoChangeMonitor and DocReposUpdated command.

Signed-off-by: Frank Kotsianas <frankkotsianas@gmail.com>
Removes doc_repos_updated command from README.

Signed-off-by: Frank Kotsianas <frankkotsianas@gmail.com>
Removes doc_repo_updated command references from bookbinder.rb and cli.rb. [Finishes #83146994].

Signed-off-by: Frank Kotsianas <frankkotsianas@gmail.com>
Merge branch 'master' of https://github.com/krujos/bookbinder