reset command: provide convenient and meaningful options help

reset command: provide convenient and meaningful options help

This commit changes the jgit "reset" command line options help from
this:

jgit reset name [VAL ...] [-- path ... ...] [--hard] [--help (-h)]
[--mixed] [--soft]
 name        : Reset current HEAD to the specified state
 [...]

to this:

jgit reset [commit-ish] [path ... ...] [-- path ... ...] [--hard]
[--help (-h)] [--mixed] [--soft]
 commit-ish  : Reset to given reference name
 [...]

Bug: 484951
Change-Id: I614e71101b4f9f46ef8f02379d1a9d135f3292d2
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>