manifest_get_default_revision fails if branch contains a '/'

[Scott Little]

Brief Description
-----------------
The manifest_get_default_revision function fails if the default branch
contains a '/'.

This was encountered while processing the delayed branching of stx-docs git
on the r/stx.6.0 release.

Severity
--------
Minor

Steps to Reproduce
------------------
From r/stx.6.0 branch ...

source $MY_REPO/build-tools/repo-utils.sh
manifest_get_default_revision

Expected Behavior
------------------
function returns r/stx.6.0

Actual Behavior
----------------
function returns r

Reproducibility
---------------
100%

System Configuration
--------------------
N/A

Branch/Pull Time/Commit
-----------------------
branch r/stx.6.0

Last Pass
---------
n/a

Timestamp/Logs
--------------
n/a

Test Activity
-------------
Product release activities

Workaround
----------
Manual branching and manifest update.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/root/+/826932

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/root/+/826932
Committed: https://opendev.org/starlingx/root/commit/955ec0af14d547523df4998009e87cec1f94e6f9
Submitter: "Zuul (22348)"
Branch: master

commit 955ec0af14d547523df4998009e87cec1f94e6f9
Author: Scott Little <email address hidden>
Date: Fri Jan 28 14:21:52 2022 -0500

    manifest_get_default_revision must deal with / in branch names

    When the manifest default revision is 'r/stx.6.0',
    manifest_get_default_revision was returning 'r'.
    A sed clause intended to catch '/>' was grabbing the wrong
    '/' character.

    Split the sed expression into two seperate expressions.

    Tested with:
       <default revision="r/stx.6.0" remote="stx-staging"/>
       <default revision="stx.6.0" remote="stx-staging"/>
       <default revision="r/stx.6.0"/>
       <default revision="stx.6.0"/>

    Closes-Bug: 1959444
    Change-Id: I3a271f6f89cea94898598c90ed2d2be59ddc60d5
    Signed-off-by: Scott Little <email address hidden>