diffstat: make --git work properly on renames (issue6025)

`$ hg diff --stat --git` shows only the new filename on renames.
I added the old filename also to the output to make it identical
with the output of `$ git diff --stat`.

Differential Revision: https://phab.mercurial-scm.org/D5628

mercurial/patch.py

@@ -2805,6 +2805,10 b' def diffstatdata(lines):'
         elif (line.startswith('GIT binary patch') or
               line.startswith('Binary file')):
             isbinary = True
+        elif line.startswith('rename from'):
+            filename = line.split()[-1]
+        elif line.startswith('rename to'):
+            filename += ' => %s' % line.split()[-1]
     addresult()
     return results
 

tests/test-diffstat.t

@@ -236,3 +236,22 b" and starting with '++' should count as a"
   $ hg diff --root . --stat
    file |  2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)
+
+When a file is renamed, --git shouldn't loss the info about old file
+  $ hg init issue6025
+  $ cd issue6025
+  $ echo > a
+  $ hg ci -Am 'add a'
+  adding a
+  $ hg mv a b
+  $ hg diff --git
+  diff --git a/a b/b
+  rename from a
+  rename to b
+  $ hg diff --stat
+   a |  1 -
+   b |  1 +
+   2 files changed, 1 insertions(+), 1 deletions(-)
+  $ hg diff --stat --git
+   a => b |  0 
+   1 files changed, 0 insertions(+), 0 deletions(-)