The /backport command today supports two arguments - "/backport repo branch". In the case that the branch is the master branch then a user only has to type "/backport repo". However if the user is backporting to a branch in the same repository as the commit they are commenting on is residing in then the command becomes the somewhat cumbersome "/backport repo branch" (it is not possible to type just /backport branch).

One could argue for a solution where the /backport command tries to figure out if the user means a repository or a branch when only the short form is used ("/backport arg"). While doable (for example by checking if a repository named arg even exists) the command becomes ambiguous if we ever have a repository and branch named "arg" (we might not have today but potentially tomorrow).

A better approach is to make the /backport command support a single argument of the form "repo:branch". This is fully backwards with the current arguments since a repository name can never contain a colon (":"). To make it slightly more convenient to backport a commit to a branch in the same repository as the commit is residing in we would allow short form "/backport :branch" (this is also fully backwards compatible and cannot become ambiguous in the future).