Merge branch 'jk/interpret-branch-name-fix' into maint

A handful of bugs around interpreting $branch@{upstream} notation
and its lookalike, when $branch part has interesting characters,
e.g. "@", and ":", have been fixed.

* jk/interpret-branch-name-fix:
  interpret_branch_name: find all possible @-marks
  interpret_branch_name: avoid @{upstream} past colon
  interpret_branch_name: always respect "namelen" parameter
  interpret_branch_name: rename "cp" variable to "at"
  interpret_branch_name: factor out upstream handling