tests/test-1102

resolve: do not crash on empty mergestate

I managed to get an empty .hg/merge/state file by interrupting a merge
by pressing Control-C. This lead to this error:

  TypeError: a2b_hex() argument 1 must be string or read-only buffer,
  not None

since localnode is assigned None before the iteration over lines in
the mergestate begins.

#!/bin/sh

rm -rf a
hg init a
cd a
echo a > a
hg ci -Am0
hg tag t1 # 1
hg tag --remove t1 # 2

hg co 1
hg tag -r0 t1
hg tags