RFR: 1690: Make jcheck cli able to use conf from workspace or other commit [v3]

[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`

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  implemented case 5

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1428/files
  - new: https://git.openjdk.org/skara/pull/1428/files/b636c1b5..5c5ce30c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1428&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1428&range=01-02

  Stats: 122 lines in 7 files changed: 109 ins; 5 del; 8 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