[PATCH 4 of 5] update: fix typo/stale comment to match code

[Martin von Zweigbergk]

# HG changeset patch
# User Martin von Zweigbergk <martinvonz at google.com>
# Date 1486662931 28800
#      Thu Feb 09 09:55:31 2017 -0800
# Node ID 05ff0729e9f4166f1f08855d25aa6ed4dddf155b
# Parent  85fdf08596df69b9e9d8dd206e90d9017416b572
update: fix typo/stale comment to match code

The comment about "obsolete.background" seems to have been about
obsolete.foreground ever since it was introduced in a59e575c6ff8
(update: allow dirty update to foreground (successors), 2013-04-16),
so let's change it.

diff -r 85fdf08596df -r 05ff0729e9f4 mercurial/merge.py
--- a/mercurial/merge.py	Wed Feb 08 23:03:33 2017 -0800
+++ b/mercurial/merge.py	Thu Feb 09 09:55:31 2017 -0800
@@ -1554,7 +1554,7 @@
                 dirty = wc.dirty(missing=True)
                 if dirty:
                     # Branching is a bit strange to ensure we do the minimal
-                    # amount of call to obsolete.background.
+                    # amount of call to obsolete.foreground.
                     foreground = obsolete.foreground(repo, [p1.node()])
                     # note: the <node> variable contains a random identifier
                     if repo[node].node() in foreground: