sha1_name: remove upstream_mark()

The first caller get_sha1_basic() just wants to make sure that no
non-numerical @{...} form was matched, so that it can proceed with
processing numerical @{...} forms.  Since we're going to introduce more
non-numerical @{...} forms, replace this upstream_mark() call with a
call to at_mark() passing NULL as the last argument; we don't care what
the kind is: all we need to know is if the return value is zero (parse
failure).

The second caller interpret_branch_name() will be expanded in the future
to handle all possible AT_KIND_* values.  So, replace the
upstream_mark() call with an upstream_mark() call capturing at_kind and
using it in a switch statement to perform the appropriate action.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
sha1_name: prepare to introduce AT_KIND_PUSH

Introduce an AT_KIND_PUSH to be represented as "@{p[ush]}".  Determine
it using branch.remote.push_refspec.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>