Reconsider revert/patch project

[auneri]

Standard ExternalProject behavior is Download > Update > Patch.

• ✗ patching is applied only once, conflicts with update.

RevertProject, when

• source exists (after download), and
• patch file is updated, or
• update is required.

PatchProject, when

• RevertProject step is out of date.
• ✗ updating patched projects implies re-patching.

UpdateProject?

• ✓ an update command that does trigger dependent steps if not needed.
• ✓ should check for git/svn as well as tar/zip.