Replace 'committish' in documentation and comments with 'commit-ish'
to match gitglossary(7) and to be consistent with 'tree-ish'.

The only remaining instances of 'committish' are:
  * variable, function, and macro names
  * "(also committish)" in the definition of commit-ish in
    gitglossary[7]
Signed-off-by: NRichard Hansen <rhansen@bbn.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>