fix: various improvements to get-cache

[eric-wieser]

Apologies for the size of this PR; my impression was that the semantics of some of the commands were mixed up, and it was easier to change them all at once

Before this PR

• leanproject mk-cache --force on a dirty tree:

  • Creates a cache entry for HEAD that represents the dirty state of the working tree (which is not HEAD!)

• leanproject get-cache

  • on Mathlib:
    • Does a full history search
    • Unpacks the full archive over the working tree. If this archive comes from azure, it contains only oleans; if it comes from mk-cache it contains all the lean source files too.
  • on another project:
    • Uses only an exact match
    • Always overwrites local lean files, meaning that get-cache --rev old_revision acts much like git checkout old_revision -- .

• leanproject get-mathlib-cache

  • on mathlib:
    • Behaved identically to leanproject get-cache
  • on another project:
    • Does a full history search
    • Checks out lean files by running git reset on mathlib

After this PR

• leanproject mk-cache --force on a dirty tree:

  • is not supported. --force is now only for overwriting the cache if it exists

• leanproject get-cache on any project:

  • Does a full history search
  • Unpacks only olean files
  • No longer supports --force. It only made sense in the presence of the destructive behavior regarding .lean files, which is no longer present.

• leanproject get-mathlib-cache

  • No longer supports --rev
  • on mathlib:
    • Behaved identically to leanproject get-cache, spits out a warning telling the user to use get-cache directly.
  • on another project:
    • Does a full history search
    • Checks out mathlib lean files by running git reset on mathlib, then applying lean files from the tarball.

This replaces #96