pull: don't suggest running hg merge when new heads are on different branches
After a pull when new heads are added but no head is added on the current
branch, the "run 'hg merge'" message can be misleading. This patch doesn't
output the merge message in that scenario.
test that 'hg commit' does not crash if the user removes a newly added file
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ touch b
$ hg add b
$ rm b
$ hg commit -A -m"comment #1"
removing b
nothing changed
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