git clone and Launchpad build system

[Joseph Rushton Wakeling]

Is there an issue with using git source for parts of snap packages when using 
the Launchpad build system?  I'm having consistent build failures for this; 
originally I tried switching from `git://` URLS to `https://`, but I'm still 
seeing the same errors, see e.g. 
https://launchpadlibrarian.net/311390659/buildlog_snap_ubuntu_xenial_amd64_ldc2_BUILDING.txt.gz:

Command '['git', 'clone', '--recursive', '--branch', 'v0.17.3', 
'https://github.com/ldc-developers/ldc.git', 
'/build/ldc2/parts/ldc-bootstrap/src']' returned non-zero exit status 128
Traceback (most recent call last):
   File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 202, in main
     builder.pull()
   File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 139, in pull
     env=env)
   File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 92, in 
run_build_command
     self.chroot(["/bin/sh", "-c", command], echo=echo)
   File "/usr/share/launchpad-buildd/slavebin/buildsnap", line 66, in chroot
     "/usr/bin/sudo", "/usr/sbin/chroot", self.chroot_path] + args)
   File "/usr/lib/python2.7/subprocess.py", line 541, in check_call
     raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '['/usr/bin/sudo', '/usr/sbin/chroot', 
'/home/buildd/build-SNAPBUILD-28577/chroot-autobuild', 'linux64', '/bin/sh', 
'-c', 'cd /build/ldc2 && env LANG=C.UTF-8 
https_proxy=http://snap-proxy.launchpad.net:3128 SNAPCRAFT_SETUP_CORE=1 
http_proxy=http://snap-proxy.launchpad.net:3128 SNAPCRAFT_LOCAL_SOURCES=1 
snapcraft pull']' returned non-zero exit status 1
Revoking proxy token...
RUN: /usr/share/launchpad-buildd/slavebin/scan-for-processes 
['scan-for-processes', 'SNAPBUILD-28577']
Scanning for processes to kill in build 
/home/buildd/build-SNAPBUILD-28577/chroot-autobuild...
RUN: /usr/share/launchpad-buildd/slavebin/umount-chroot ['umount-chroot', 
'SNAPBUILD-28577']
Unmounting chroot for build SNAPBUILD-28577...
RUN: /usr/share/launchpad-buildd/slavebin/remove-build ['remove-build', 
'SNAPBUILD-28577']

The corresponding part of the snapcraft.yaml:
https://github.com/ldc-developers/ldc2.snap/blob/da18a99f4c66c219b4cf9d06b4087a5fe9acce79/snap/snapcraft.yaml#L53-L55

   ldc-bootstrap:
     source: https://github.com/ldc-developers/ldc.git
     source-tag: v0.17.3