diff: make sure we output stat even when --git is not passed (issue4037) (BC)

diff: make sure we output stat even when --git is not passed (issue4037) (BC)

Before this patch, `hg diff --stat` will give an empty output. It will not show
the stat information. I debugged and found that the underlying code does not
return the diff header and due to that, other code paths fails to parse that as
a diff.

I looked into why we don't return diff headers in quiet mode and found the
behavior is from 8f8bb77d560e70bcc95577e4dfa877df18d876ab which does not have
any mention about why it is done. We also show the diff headers in git, so I
think it's fine showing diff header in normal diff in quiet mode.

Differential Revision: https://phab.mercurial-scm.org/D6007

--- a/mercurial/patch.py	Sat Feb 23 04:50:46 2019 +0530
+++ b/mercurial/patch.py	Fri Feb 22 03:52:10 2019 +0530
@@ -2682,7 +2682,7 @@
                         header.append('similarity index %d%%' % sim)
                     header.append('%s from %s' % (copyop, path1))
                     header.append('%s to %s' % (copyop, path2))
-        elif revs and not repo.ui.quiet:
+        elif revs:
             header.append(diffline(path1, revs))
 
         #  fctx.is  | diffopts                | what to   | is fctx.data()

--- a/tests/test-commit.t	Sat Feb 23 04:50:46 2019 +0530
+++ b/tests/test-commit.t	Fri Feb 22 03:52:10 2019 +0530
@@ -512,6 +512,7 @@
   HG: dels=
   HG: files=changed
   HG:
+  HG: diff -r d2313f97106f changed
   HG: --- a/changed	Thu Jan 01 00:00:00 1970 +0000
   HG: +++ b/changed	Thu Jan 01 00:00:00 1970 +0000
   HG: @@ -1,1 +1,2 @@
@@ -573,6 +574,7 @@
   HG: dels=removed
   HG: files=added removed
   HG:
+  HG: diff -r d2313f97106f added
   HG: --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   HG: +++ b/added	Thu Jan 01 00:00:00 1970 +0000
   HG: @@ -0,0 +1,1 @@
@@ -583,6 +585,7 @@
   HG: dels=removed
   HG: files=added removed
   HG:
+  HG: diff -r d2313f97106f removed
   HG: --- a/removed	Thu Jan 01 00:00:00 1970 +0000
   HG: +++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
   HG: @@ -1,1 +0,0 @@

--- a/tests/test-diff-hashes.t	Sat Feb 23 04:50:46 2019 +0530
+++ b/tests/test-diff-hashes.t	Fri Feb 22 03:52:10 2019 +0530
@@ -13,6 +13,7 @@
   $ hg ci -m 'change foo'
 
   $ hg --quiet diff -r 0 -r 1
+  diff -r a99fb63adac3 -r 9b8568d3af2f foo
   --- a/foo	Thu Jan 01 00:00:00 1970 +0000
   +++ b/foo	Thu Jan 01 00:00:00 1970 +0000
   @@ -1,1 +1,1 @@

--- a/tests/test-diffstat.t	Sat Feb 23 04:50:46 2019 +0530
+++ b/tests/test-diffstat.t	Fri Feb 22 03:52:10 2019 +0530
@@ -289,3 +289,6 @@
   A new c
   R c
   $ hg diff --stat -q
+   c     |  1 -
+   new c |  1 +
+   2 files changed, 1 insertions(+), 1 deletions(-)

--- a/tests/test-mq.t	Sat Feb 23 04:50:46 2019 +0530
+++ b/tests/test-mq.t	Fri Feb 22 03:52:10 2019 +0530
@@ -305,6 +305,7 @@
 working dir diff:
 
   $ hg diff --nodates -q
+  diff -r dde259bd5934 a
   --- a/a
   +++ b/a
   @@ -1,1 +1,2 @@

--- a/tests/test-transplant.t	Sat Feb 23 04:50:46 2019 +0530
+++ b/tests/test-transplant.t	Fri Feb 22 03:52:10 2019 +0530
@@ -599,6 +599,7 @@
   > EOF
   0:17ab29e464c6
   apply changeset? [ynmpcq?]: p
+  diff -r 000000000000 -r 17ab29e464c6 r1
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   +++ b/r1	Thu Jan 01 00:00:00 1970 +0000
   @@ -0,0 +1,1 @@