Use CommitConfig.CleanupMode to implement git commit --cleanup. Add
setters for the clean-up mode, the comment character, and for the
default default clean-up mode.
Behavior of existing client code is unchanged as the default clean-up
mode is set to "verbatim". To use git config defaults, one can call
setCleanupMode(CleanupMode.DEFAULT). The default comment character
is hard-coded as '#' for now, as in other parts of JGit. Implementing
full support for core.commentChar shall be done in a separate change.
Bug: 553065
Change-Id: I470785e464a762d3f409f163f1cbdbb98dd81aaf
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>