Extract subtrees from a git repository(s) and move them into a new or
othe pre-existing repository.

split-repo.sh new-repo.map [branch]

The map file has four parts per line separated by a pipe ('|') character.
The four fields are:
1) Path to the root of the source repository
2) Relative path to the subdirectory within the source repository that is
   to be moved
3) Path to the roo of the destination repository. If non-existant, a new repo
   will be created.
4) Relative path under the destination repository where the relocated subtree
   is to be placed.

e.g.
  cat new-repo.map
  stx/stx-integ|base/centos-release-config|stx/stx-config-files|centos-release-config
  stx/stx-integ|base/dhcp-config|stx/stx-config-files|dhcp-config
  stx/stx-integ|utilities/build-info|stx/stx-utilities|utilities/build-info
  stx/stx-config|pm-qos-mgr|stx/stx-utilities|utilities/pm-qos-mgr

This will create repos stx-config-files and stx-utilities. Subdirectories
base/centos-release-config and base/dhcp-config are moved from stx-integ into
stx-config-files, dropping the 'base/' prefix.  Subdirectory utilities/build-info
is moved from stx-integ to stx-utilities at the same relative path.  Finally
pm-qos-mgr is moved from stx-config, under the utilities subdirectory of 
stx-utilities.

This tool uses filter_git_history.sh from OpenStack's oslo.tools library.
It can be obtained from https://opendev.org/openstack/oslo.tools.git.
Clone it, and set environment variable OSLO_TOOLS pointing to the top 
directory of the oslo.tools repo.