contrib: script to create a new vendor branch

This script is intended to create a new vendor branch.  Doing so is
not completely obvious if you are not familiar with the upstream
structure, so this takes the pain out of getting it right.

It doesn't check out the branch locally, but does set everything up so
that, if you have push enabled for your vendor branches, then

  git push vendors/<vendor> <branch>

will work as expected.

Run the script as

  contrib/git-add-vendor-branch.sh <vendor>/<branch> <start-point>

the <vendor> space must have previously been set up in the way
git-fetch-vendor.sh expects.

    * git-add-vendor-branch.sh: New file.

contrib/ChangeLog

@@ -1,3 +1,7 @@
+2020-01-22  Richard Earnshaw  <rearnsha@arm.com>
+
+	* git-add-vendor-branch.sh: New file.
+
 2020-01-20  Richard Earnshaw  <rearnsha@arm.com>
 
 	* gcc-git-customization.sh: Check that user-supplied remote

contrib/git-add-vendor-branch.sh

@@ -0,0 +1,48 @@
+#! /bin/sh -e
+
+# Create a new upstream vendor branch.
+
+# Usage:
+#  contrib/git-add-vendor-branch.sh <vendor>/<branch-name> <base>
+
+usage ()
+{
+    echo "Usage:"
+    echo "  $0 <vendor>/<branch-name> <start-point>"
+    echo
+    echo "<vendor> must have already been set up using contrib/git-fetch-vendor.sh"
+    exit 1
+}
+
+if [ $# != 2 ]
+then
+    usage
+fi
+
+vendor=$(echo "$1" | sed -r "s:([^/]*)/.*$:\1:")
+branch=$(echo "$1" | sed -r "s:[^/]*/(.*)$:\1:")
+start=$2
+
+# Sanity check the new branch argument.  If there is no '/', then the
+# vendor will be the same as the entire first argument.
+if [ -z "$vendor" -o -z "$branch" -o ${vendor} = $1 ]
+then
+    usage
+fi
+
+# Check that we know about the vendor
+url=$(git config --get "remote.vendors/${vendor}.url"||true)
+if [ -z "$url" ]
+then
+    echo "Cannot locate remote data for vendor ${vendor}.  Have you set it up?"
+    exit 1
+fi
+
+git branch --no-track ${vendor}/${branch} ${start}
+git push vendors/${vendor} ${vendor}/${branch}:refs/vendors/${vendor}/heads/${branch}
+git fetch -q vendors/${vendor}
+git branch --set-upstream-to=remotes/vendors/${vendor}/${branch} ${vendor}/$branch
+echo "You are now ready to check out ${vendor}/${branch}"
+echo "To push the branch upstream, use:"
+echo
+echo "git push vendors/${vendor} ${vendor}/${branch}"