Add ability to detect file/folder deletion

[JElchison]

Test importing a file deletion event from Surround

[JElchison]

As is, file (and presumably folder) deletions are not properly detected in most cases, as the sscm ls command doesn't list deleted files.

I couldn't find a way to make sscm ls list deleted files.

Find another way to detect file/folder deletions, and add delete events to database file:
* Is there a way to get a list of files that existed at any time in the past?

More brainstorming is welcome.

This affects #23.

[JElchison]

Brainstorming...

What does sscm history report when run on a deleted file? If it gives useful history (including deletion event), then try running sscm history on all known files to detect deletions.

Ideas for listing all known files:

• Via git-fast-import's ls command?
• List all known paths/files in repo across all branches
  • Create this list via sqlite database query?

[JElchison]

Confirmed: sscm history on a deleted file does provide history including the delete event.

[JElchison]

Change find_all_files_in_branch_under_path() to find_all_files_under_path() and pass in list of branches.