combo-layer: partial import for '--history init'

The new "since_revision" property can be used to cut off the imported
history at some point. This is useful to keep the resulting repository
smaller while still preserving enough history that "git annotate"
reports the right author and commit for most lines.

The initial, squashed import commit shows up with "unknown" as author
in the "git annotate" output. It has the repository name as prefix
in the subject line; importing that commit works best with a
layer hook which does not add the repository name again when
it is already present. Adding it here is useful for hooks
which do not extend the subject line.

(From OE-Core rev: 74f4c9e3bcdb3c4ca919623086e92a9379bd81ff)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/combo-layer.conf.example

@@ -63,11 +63,24 @@ last_revision =
 # example:
 #     hook = combo-layer-hook-default.sh
 
+# since_revision:
+#   since_revision = release-1-2
+#   since_revision = 12345 abcdf
+#
+# If provided, truncate imported history during "combo-layer --history
+# init" at the specified revision(s).  More than one can be specified
+# to cut off multiple component branches.
+#
+# The specified commits themselves do not get imported. Instead, an
+# artificial commit with "unknown" author is created with a content
+# that matches the original commit.
+
 [oe-core]
 src_uri = git://git.openembedded.org/openembedded-core
 local_repo_dir = /home/kyu3/src/test/oecore
 dest_dir = .
 last_revision =
+since_revision = some-tag-or-commit-on-master-branch
 
 # It is also possible to embed python code in the config values. Similar
 # to bitbake it considers every value starting with @ to be a python