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.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..