convert: avoid updating tags when there is nothing new
Previously, when converting from a mercurial repo there would be an extraneous
commit at the end of the convert process that would rewrite tags. Now, we check
if there are any new tags before doing this rewriting.
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 0 ..... 0 b80de5d13875 000000000000 000000000000 (re)
1 0 24 ..... 1 0376abec49b8 000000000000 000000000000 (re)
$ cd ..