git-commit: log parameter updates.

git-commit: log parameter updates.

While moving '-m' to make room for CVS compatible "here is the
log message", enhance source of log parameters.

  -m 'message': a command line parameter.
  -F <file>   : a file (use '-' to read from stdin).
  -C <commit> : message in existing commit.
  -c <commit> : message in existing commit (allows further editing).

Longer option names for these options are also available.

While we are at it, get rid of shell array bashism.

Signed-off-by: Junio C Hamano <junkio@cox.net>