[devtool] add branch argument to release cmd
[devtool] add branch argument to release cmd

This commit also modifies POST API json to not include
`target_commitish` field. According to GitHub docs on release API,
the commitish field is unused if the Git tag already exists.
This is always the case for us, because posting a draft release
is preceded by tag creation.

Signed-off-by: Luminita Voicu <6c9c51795b037b189e54c262e37c535895f48539@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker