filemerge: when not keeping premerge, don't write markers to context
When premerge is enabled (as it is for non-binary inputs by default)
and the markers are not kept, we currently still write it to the
output context and then restore the previous content right after. With
the refactoring in the previous patch, we can easily avoid that step
and instead write the output in the opposite case (i.e. when it's
successful or when the markers are supposed to be kept).
Differential Revision: https://phab.mercurial-scm.org/D12149
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present