Bugfix: Only use git for build info if the repository is actually the right one

Also adds ability to disable check with BITCOIN_GENBUILD_NO_GIT=1 in the environment

contrib/gitian-descriptors/gitian-linux.yml

@@ -70,6 +70,7 @@ script: |
   done
 
   # Create the release tarball using (arbitrarily) the first host
+  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   ./configure --prefix=${BASEPREFIX}/`echo "${HOSTS}" | awk '{print $1;}'`
   make dist

contrib/gitian-descriptors/gitian-osx.yml

@@ -80,6 +80,7 @@ script: |
   done
 
   # Create the release tarball using (arbitrarily) the first host
+  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   ./configure --prefix=${BASEPREFIX}/`echo "${HOSTS}" | awk '{print $1;}'`
   make dist

contrib/gitian-descriptors/gitian-win.yml

@@ -73,6 +73,7 @@ script: |
   done
 
   # Create the release tarball using (arbitrarily) the first host
+  export GIT_DIR="$PWD/.git"
   ./autogen.sh
   ./configure --prefix=${BASEPREFIX}/`echo "${HOSTS}" | awk '{print $1;}'`
   make dist