[Packaging] Introduce gbp-flat-tree.sh to simplify the Tizen build process.

TL;DR: one can simply do the following to build an RPM:
  cd /path/to/src/xwalk
  gbs build -A i586

We can get rid of the need for a separate git tree and the
generate-flat-tree.sh script by relying on the hooks provided by
git-buildpackage: the postexport hook that we use is called right after the
git repository's sources have been exported with `git archive`.

In this hook, we execute a new script, `gbp-flat-tree.sh`. It does pretty
much the same thing as generate-flat-script.sh, but automatically. That is,
it removes the tarball generated by `git archive` (since gbs build should be
called from xwalk/, it would only contain Crosswalk's contents anyway) and
replaces it with a new one with the contents of src/.

Since it is called at a stage in the whole build process that immediately
precedes the call to `/usr/bin/build`, we do not need to keep a separate
flat git tree with all our sources anymore; we are effectively bypassing
git-buildpackage's default export mechanism.

The downside of this approach (which is shared by the previous one with
`generate-flat-tree.sh`, anyway) is that all files in src/ are archived,
even if they are not part of any commit. In other words, we end up acting as
if "--include-all" was always passed to `gbs build`.

(Fun fact: the whole dance of calling `gclient recursive` twice with
different arguments and everything was unnecessary; one of the entries
reported by those calls is src/ itself, so we always ended up archiving
everything under it)

Author: Raphael Kubo da Costa

.gbp.conf

@@ -0,0 +1,12 @@
+# For more information about this file, see git-buildpackage's documentation.
+# It is used by gbs in order to build Crosswalk packages for Tizen.
+
+[DEFAULT]
+# This postexport hook is used to replace the default crosswalk.tar file
+# generated with `git archive' (and which only contains the contents of
+# crosswalk.git) with a full-blown tar file that contains all source code
+# required to build Crosswalk. Additionally, the new tar file's members all
+# contain their original mtimes, which is helpful if one wants to try an
+# incremental Tizen build. The script is run from the temporary directory
+# containing the contents of packaging/ that are used by git-buildpackage.
+postexport = ./gbp-flat-tree.sh

packaging/gbp-flat-tree.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+
+# Copyright (c) 2013 Intel Corporation. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This script is expected to be run by `git-buildpackage' (called by `gbs
+# build') as a postexport hook. Its purpose is to remove the tar file generated
+# by `gbs export' with `git archive' and create a new archive with tar itself.
+#
+# It is helpful in two ways:
+# - It automates the generation of a proper archive for RPM builds so that one
+#   does not need to create a separate git tree with all sources for `gbs
+#   build' to work.
+# - Since tar is used, the archive's members all have their actual mtimes as
+#   opposed to the time of the git tree-ish passed to `git archive'. This is
+#   part of the solution for incremental builds in Tizen: since we use actual
+#   file mtimes, they are not rebuilt by `make'.
+#
+# As a postexport hook, there are two additional environment variables
+# available: GBP_GIT_DIR is /path/to/src/xwalk/.git, and GBP_TMP_DIR is the
+# temporary directory containing everything in /path/to/src/xwalk/packaging
+# that will be copied to $GBSROOT/local/sources and used by `rpmbuild' to build
+# an RPM package.
+#
+# The script is run from GBP_TMP_DIR.
+
+# Fail early to avoid bigger problems later in the process.
+set -e
+
+VERSION_NUMBER=`awk '/^Version:/ { print $2 }' crosswalk.spec`
+TAR_FILE="${GBP_TMP_DIR}/crosswalk-${VERSION_NUMBER}.tar"
+
+if [ ! -f "${TAR_FILE}" ]; then
+    echo "${TAR_FILE} does not exist. Aborting."
+    exit 1
+fi
+
+# The top-level directory that _contains_ src/.
+BASE_SRC_DIR=`readlink -f "${GBP_GIT_DIR}/../../.."`
+if [ $? -ne 0 ]; then
+    echo "${GBP_GIT_DIR}/../../.. does not seem to be a valid path. Aborting."
+    exit 1
+fi
+
+# Erase the archive generated with `git archive'.
+rm -v "${TAR_FILE}"
+
+echo "Creating a new ${TAR_FILE} from ${BASE_SRC_DIR}/src"
+
+# The --transform parameter is used to prepend all archive members with
+# crosswalk/ so they all share a common root. Note it is crosswalk/, without
+# any version numbers, so that any build files in an external directory
+# referring to a source file does not need to be updated just because of a
+# version bump.
+tar --update --file "${TAR_FILE}" \
+    --exclude-vcs --exclude=native_client --exclude=LayoutTests \
+    --exclude=src/out --directory="${BASE_SRC_DIR}" \
+    --transform="s:^:crosswalk-${VERSION_NUMBER}/:S" \
+    src