RFR: 1690: Make jcheck cli able to use conf from workspace or other commit

[Zhao Song]

Currently, the command `git skara jcheck` can only use the `.jcheck/conf` configuration file in the current checking commit.
However, sometimes users want to run jcheck with different configurations to validate their commits. Therefore, we need to upgrade `git skara jcheck`.

In this patch, `git skara jcheck` would be able to support following usecases.

1. Run jcheck on a commit or a series of commits using the .jcheck/conf in the same commit. (what we do by default today)
`git skara jcheck`

2. Run jcheck on a commit or a series of commits using the .jcheck/conf in a different specified commit. 
`git skara jcheck --specified-conf-commit <COMMIT HASH>`

3. Run jcheck on a commit or a series of commits using the .jcheck/conf in my workspace. 
`git skara jcheck --workspace-conf`

4. Run jcheck on a commit or a series of commits using a config file that I point to directly, that may have any name. 
`git skara jcheck --workspace-conf --conf-file <FILENAME>`

5. Run jcheck on the diff in my current workspace, either --staged or not using the .jcheck/conf in my workspace.
`git skara jcheck --workspace-diff`

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Commit messages:
 - SKARA-1690

Changes: https://git.openjdk.org/skara/pull/1428/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1428&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1690
  Stats: 108 lines in 7 files changed: 103 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1428.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1428/head:pull/1428

PR: https://git.openjdk.org/skara/pull/1428