Merge branch 'maint'

* maint:
  Update draft release notes for 1.6.0.1
  Add hints to revert documentation about other ways to undo changes
  Install templates with the user and group of the installing personality
  "git-merge": allow fast-forwarding in a stat-dirty tree
  completion: find out supported merge strategies correctly
  decorate: allow const objects to be decorated
  for-each-ref: cope with tags with incomplete lines
  diff --check: do not get confused by new blank lines in the middle
  remote.c: remove useless if-before-free test
  mailinfo: avoid violating strbuf assertion
  git format-patch: avoid underrun when format.headers is empty or all NLs

diff.c

@@ -1628,6 +1628,7 @@ static void builtin_checkdiff(const char *name_a, const char *name_b,
 		xdemitcb_t ecb;
 
 		memset(&xecfg, 0, sizeof(xecfg));
+		xecfg.ctxlen = 1; /* at least one context line */
 		xpp.flags = XDF_NEED_MINIMAL;
 		xdi_diff_outf(&mf1, &mf2, checkdiff_consume, &data,
 			      &xpp, &xecfg, &ecb);