Give proper error message when refusing to overwrite existing branch

If fast-export was asked to export a Mercurial branch to Git and a
branch of the same name already existed in the Git repo but it was not
created by fast export, fast-export would crash while trying to format
an error message claiming that the destination branch was modified
behind its back.

This patch extends fast-export to detect the situation above and give
a proper error message which hopefully is less confusing to the user.

Credits for discovering the original crash goes to Shun-ichi Goto
<gotoh@taiyo.co.jp>.

Closes: #269.

hg-fast-export.py

@@ -493,7 +493,12 @@ def verify_heads(ui,repo,cache,force,ignore_unnamed_heads,branchesmap):
     sanitized_name=sanitize_name(b,"branch",branchesmap)
     sha1=get_git_sha1(sanitized_name)
     c=cache.get(sanitized_name)
-    if sha1!=c:
+    if not c and sha1:
+      stderr_buffer.write(
+        b'Error: Branch [%s] already exists and was not created by hg-fast-export, '
+        b'export would overwrite unrelated branch\n' % b)
+      if not force: return False
+    elif sha1!=c:
       stderr_buffer.write(
         b'Error: Branch [%s] modified outside hg-fast-export:'
         b'\n%s (repo) != %s (cache)\n' % (b, b'<None>' if sha1 is None else sha1, c)