merge: print status message before launching external merge tool
It seems somewhat common that people run into a merge conflict and
don't notice the launched merge tool, and instead they think hg just
hung. Let's print a message for each file that we launch a GUI merge
tool for.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a