RFR: 1690: Make jcheck cli able to use conf from workspace or other commit [v2]

[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 three additional commits since the last revision:

 - Merge remote-tracking branch 'origin/skara-1690' into skara-1690
   
   # Conflicts:
   #	cli/src/main/java/org/openjdk/skara/cli/GitJCheck.java
   #	jcheck/src/test/java/org/openjdk/skara/jcheck/TestRepository.java
   #	vcs/src/main/java/org/openjdk/skara/vcs/ReadOnlyRepository.java
   #	vcs/src/main/java/org/openjdk/skara/vcs/git/GitRepository.java
   #	vcs/src/main/java/org/openjdk/skara/vcs/hg/HgRepository.java
 - fix comment
 - case 5 has not been implemented

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1428/files
  - new: https://git.openjdk.org/skara/pull/1428/files/66460b94..b636c1b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1428&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1428&range=00-01

  Stats: 104 lines in 5 files changed: 50 ins; 18 del; 36 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