Teach "git reflog" a subcommand to delete single entries

Teach "git reflog" a subcommand to delete single entries

This commit implements the "delete" subcommand:

	git reflog delete master@{2}

will delete the second reflog entry of the "master" branch.

With this, it should be easy to implement "git stash pop" everybody
seems to want these days.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Shawn O. Pearce <redacted>