gitian: use latest builder and optimize git interaction

Fetching only the Monero version to be built reduces the downloaded
volume by two thirds (from ~240 MB down to ~80 MB).

While a non-issue for users on modern networks, the optimization is
critical on slow/flaky connections (read: hidden networks) where any
increase in volume makes the clone-step more likely to fail with error:
RPC failed; curl 18 transfer closed with outstanding read data remaining.

Personally I wasn't able to get past this issue on a torified-connection
at all without this patch, hence the PR.

I've confirmed (with dockrun.sh) that this builds the v0.18.3.1-linux
binaries with checksums matching those reported by other builders:
https://github.com/monero-project/gitian.sigs/tree/master/v0.18.3.1-linux

contrib/depends/builders/darwin.mk

@@ -7,7 +7,7 @@ build_darwin_OTOOL: = $(shell xcrun -f otool)
 build_darwin_NM: = $(shell xcrun -f nm)
 build_darwin_INSTALL_NAME_TOOL:=$(shell xcrun -f install_name_tool)
 build_darwin_SHA256SUM = shasum -a 256
-build_darwin_DOWNLOAD = curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
+build_darwin_DOWNLOAD = curl --location --fail --continue-at - --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
 
 #darwin host on darwin builder. overrides darwin host preferences.
 darwin_CC=$(shell xcrun -f clang) -mmacosx-version-min=$(OSX_MIN_VERSION)

contrib/depends/builders/linux.mk

@@ -1,2 +1,2 @@
 build_linux_SHA256SUM = sha256sum
-build_linux_DOWNLOAD = curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
+build_linux_DOWNLOAD = curl --location --fail --continue-at - --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o

contrib/gitian/dockrun.sh

@@ -43,8 +43,8 @@ WORKDIR $WORKDIR
 
 RUN	git clone https://github.com/monero-project/gitian.sigs.git sigs; \
   git clone https://github.com/devrandom/gitian-builder.git builder; \
-  cd builder; git checkout c0f77ca018cb5332bfd595e0aff0468f77542c23; mkdir -p inputs var; cd inputs; \
-  git clone https://github.com/monero-project/monero
+  cd builder; git checkout 6f21a0ef5c9ea3df1a515c4b8b9a9296512821dd; mkdir -p inputs var; cd inputs; \
+  git clone -v --branch $VERSION --depth 1 https://github.com/monero-project/monero
 
 CMD ["sleep", "infinity"]
 EOF

contrib/gitian/gitian-build.py

@@ -7,6 +7,7 @@ import sys
 
 gsigs = 'https://github.com/monero-project/gitian.sigs.git'
 gbrepo = 'https://github.com/devrandom/gitian-builder.git'
+gbcommit = '6f21a0ef5c9ea3df1a515c4b8b9a9296512821dd'
 
 platforms = {'l': ['Linux', 'linux', 'tar.bz2'],
         'a': ['Android', 'android', 'tar.bz2'],
@@ -28,7 +29,7 @@ def setup():
     if not os.path.isdir('builder'):
         subprocess.check_call(['git', 'clone', gbrepo, 'builder'])
     os.chdir('builder')
-    subprocess.check_call(['git', 'checkout', 'c0f77ca018cb5332bfd595e0aff0468f77542c23'])
+    subprocess.check_call(['git', 'checkout', gbcommit])
     os.makedirs('inputs', exist_ok=True)
     os.chdir('inputs')
     if os.path.isdir('monero'):
@@ -67,7 +68,7 @@ def rebuild():
 
         print('\nCompiling ' + args.version + ' ' + os_name)
         infile = 'inputs/monero/contrib/gitian/gitian-' + tag_name + '.yml'
-        subprocess.check_call(['bin/gbuild', '-j', args.jobs, '-m', args.memory, '--commit', 'monero='+args.commit, '--url', 'monero='+args.url, infile])
+        subprocess.check_call(['bin/gbuild', '--skip-fetch', '-j', args.jobs, '-m', args.memory, '--commit', 'monero='+args.commit, '--url', 'monero='+args.url, infile])
         subprocess.check_call(['bin/gsign', '-p', args.sign_prog, '--signer', args.signer, '--release', args.version+'-'+tag_name, '--destination', '../sigs/', infile])
         subprocess.check_call('mv build/out/monero-*.' + suffix + ' ../out/'+args.version, shell=True)
         print('Moving var/install.log to var/install-' + tag_name + '.log')