changelog-from-hg-log hydra build failure

[Mike Miller]

I noticed that our hydra autobuild has been failing for a while now. The
current build error is actually very trivial:

  building ChangeLog
    GEN      ChangeLog
  building distdir
  if test -d "octave-4.1.0+"; then find "octave-4.1.0+" -type d ! -perm -200 
-exec chmod u+w {} ';' && rm -rf "octave-4.1.0+" || { sleep 5 && rm -rf 
"octave-4.1.0+"; }; else :; fi
  test -d "octave-4.1.0+" || mkdir "octave-4.1.0+"
  cp: cannot stat './ChangeLog': No such file or directory
  make[1]: *** [distdir] Error 1

I think this error comes from the following change

  http://hg.savannah.gnu.org/hgweb/octave/rev/9df3feddcf22
  only generate ChangeLog if building from hg archive

Hydra apparently builds from a tarball snapshot from hg archive, not
from a full hg clone. On previous revisions, the changelog-from-hg-log
rule would simply build an empty ChangeLog file if it's not run in an hg
clone, but with this change the ChangeLog file is not generated anymore.

Was the reason for this to avoid overwriting a valid ChangeLog file in
case make decides to run the rule? Could we change this so at least an
empty ChangeLog file is created if the file doesn't already exist?

-- 
mike