Fix-Check for heads before assigning branch tips

[delacor]

branchheads() reports an empty list for some branches which result in:

Traceback (most recent call last):
  File "/home/john/git/fast-export/hg-fast-export.py", line 739, in <module>
    sys.exit(hg2git(options.repourl,m,options.marksfile,options.mappingfile,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/john/git/fast-export/hg-fast-export.py", line 556, in hg2git
    if not verify_heads(ui,repo,heads_cache,force,ignore_unnamed_heads,branchesmap):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/john/git/fast-export/hg-fast-export.py", line 494, in verify_heads
    branches[bn] = branchtip(repo, heads)
                   ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/john/git/fast-export/hg-fast-export.py", line 482, in branchtip
    tip = heads[-1]
          ~~~~~^^^^
IndexError: list index out of range

[frej]

@delacor, can you create a small test example where this error occurs or check that you get the crash for closed branches?

It may be that the change in 76db75d should be using heads = repo.branchmap().branchheads(bn, closed=True) in order to preserve the old behaviour.