title: cmake Failed to checkout tag: 'master'
body: <p>Im trying to build an open source project that uses external project.</p>
  <p>I forked the external project and I replaced these lines</p>
  <p><code>GIT_REPOSITORY    https://github.com/OpenDroneMap/OpenSfM/</code></p>
  <p><code>GIT_TAG           264</code></p>
  <p>with</p>
  <p><code>GIT_REPOSITORY    https://github.com/myname/OpenSfM/</code></p>
  <p>When i try to build, i get error below;</p>
  <pre><code>fatal: invalid reference: master
  CMake Error at /code/SuperBuild/build/opensfm/tmp/opensfm-gitclone.cmake:40 (message):
     Failed to checkout tag: 'master'
  
  make[2]: *** [CMakeFiles/opensfm.dir/build.make:110: opensfm/stamp/opensfm-download] Error 1
  make[1]: *** [CMakeFiles/Makefile2:350: CMakeFiles/opensfm.dir/all] Error 2
  </code></pre>
  <p>OpenSfM repo uses main tag for master branch.</p>
  <p>What happens if cmake fails to checkout nonexistent tag ? Does compilation continue with main branch ?</p>