Speed up pbuilder: disable building of manual pages with a hook script

Credits: Debian bug #561539
Add execute permissions to D10-man-db
Rework generate-git-snapshot to make sure quilt can't break the build

When using the dpkg 3.0 (quilt) format (see debian/source/format)
there might be quilt patches still applied in the source tree,
either because of a broken build or under certain - though not yet
clearly identified - situations which seem to end with a
"diff: standard output: Broken pipe" message in the build log.

This then might end up in a build failure like:

| + cd /var/lib/jenkins/workspace/glusterfs-source
| + dpkg-source -i -I -b source
| dpkg-source: info: using source format `3.0 (quilt)'
| diff: standard output: Broken pipe
| diff: standard output: Broken pipe
| dpkg-source: info: building glusterfs using existing ./glusterfs_3.2.7.orig.tar.gz
| dpkg-source: info: local changes detected, the modified files are:
|  source/ChangeLog
|  source/doc/gluster.8
|  source/doc/glusterfs.8
|  source/libglusterfs/src/common-utils.c
|  source/rpc/rpc-lib/src/rpcsvc.c
|  source/xlators/mgmt/glusterd/src/glusterd-rpc-ops.c
| dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/glusterfs_3.2.7-2~bpo60+1.diff.3Wkisr
| dpkg-source: info: you can integrate the local changes with dpkg-source --commit

While everything has been cleanly integrated in debian/patches/
the build process fails because it looks like there have been
modifications to upstream sources whereas it's just wrong
information about patches applied.

So instead make sure we enter a clean git repository before doing
any changes (this can be skipped via SKIP_GIT_CLEANUP=true) and
also unapply all present quilt patches (this can be skipped via
SKIP_QUILT_CLEANUP=true). Finally replace the dpkg-source call
with dpkg-buildpackage and its --source-option=--unapply-patches
feature. While the combination of the git cleanup, quilt cleanup
and dpkg-source->dpkg-buildpackage is more than explicitely
necessary this provides more flexibility if needed.

Thanks: Raphael Hertzog <hertzog@debian.org> for his dpkg related feedback
Remove *_source.changes file generated by dpkg-buildpackage in *-source job [Closes: #67]

The recent commit to make sure quilt can't break the build (git
commit 63d6c47) made a switch from dpkg-source to
dpkg-buildpackage. dpkg-buildpackage generates a *_source.changes
file which isn't present when using dpkg-source. While this file
is nice it doesn't provide any real value for us, but what's more
important, it might break existing setups:

If all files are copied around from one Jenkins job to another
then in the final jenkins job the changes file from the binaries
job and the source.changes one end up together. If those are
passed to other tools then things might break (see github issue
67 as an example). So let's just remove the file after the
dpkg-buildpackage call to provide the used behaviour.
build-and-provide-package: Look only for the "real" debian/control file

Otherwise this breaks in a situation like that:

| % tar atf autopkgtest_2.3.5~pgdg70+1.tar.gz | grep control
| -rw-r--r-- 0/0             297 2013-09-25 08:51 source/tests/testpkg/debian/control
| -rw-r--r-- 0/0             151 2013-09-25 08:51 source/debian/tests/control
| -rw-r--r-- 0/0            1982 2013-09-25 08:51 source/debian/control

Thanks: Christoph Berg <christoph.berg@credativ.de> for the bugreport and fix