Show usage string for 'git commit-tree -h'

Show usage string for 'git commit-tree -h'

Treat an "-h" option as a request for help, rather than a "Not a
valid object name" error.

"commit-tree -h" could be asking to create a new commit from a
treeish named "-h".  Strictly speaking, such a pathological ref
name is possible, but the user would have to had said something
like "tags/-h" to name such a pathological already.  commit-tree
is usually used in scripts with raw object ids, anyway.

For consistency, the "-h" option uses its new meaning even if
followed by other arguments.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>