simplemerge: move printing of merge result to extension
The `mercurial.simplemerge` module still has some command-lines
processing that doesn't belong in such a low-level module. This patch
moves the handling of `hg simplemerge --print` to the extension by
having `mercurial.simplemerge.simplemerge()` return the merged text.
Differential Revision: https://phab.mercurial-scm.org/D12148
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3