Expose upstream and dest-branch attributes through environment

Recent changes in ChromeOS Infra to ensure we're reading from
snapshot manifests properly have exposed several bugs in our
assumptions about manifest files.  Mainly that the revision field
for a project does _not_ have to refer to a ref, it can just be
a commit hash.

Several places assume that the revision field can be parsed as a
ref to get the branch the project is on, which isn't true.  To fix
this we need to be able to look at the upstream and dest-branch
attributes of the repo, so we expose them through the environment
variables set in `repo forall`.

Test: manual 'repo forall' run
Bug: https://crbug.com/1032441

Change-Id: I2c039e0f4b2e0f430602932e91b782edb6f9b1ed
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/263132
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Sean McAllister <smcallis@google.com>