git --diff: fix traceback when getting mode change

- use the manifest instead of the mode in the working dir
  if the diff is against two revisions
- add a testcase

mercurial/patch.py

@@ -502,7 +502,10 b' def diff(repo, node1=None, node2=None, f'
                     header.append('deleted file mode %s\n' % mode)
             else:
                 omode = gitmode(mmap.execf(f))
-                nmode = gitmode(util.is_exec(repo.wjoin(f), mmap.execf(f)))
+                if node2:
+                    nmode = gitmode(mmap2.execf(f))
+                else:
+                    nmode = gitmode(util.is_exec(repo.wjoin(f), mmap.execf(f)))
                 addmodehdr(header, omode, nmode)
             r = None
             if dodiff:

tests/test-git-export

@@ -50,3 +50,7 b" hg ci -mrenamemod -d '0 0'"
 echo '% rename+mod+chmod'
 hg diff --git -r 6:tip | sed -e "s/\(+++ [a-zA-Z0-9_/.-]*\).*/\1/" \
     -e "s/\(--- [a-zA-Z0-9_/.-]*\).*/\1/"
+
+echo '% nonexistent in tip+chmod'
+hg diff --git -r 5:6 | sed -e "s/\(+++ [a-zA-Z0-9_/.-]*\).*/\1/" \
+    -e "s/\(--- [a-zA-Z0-9_/.-]*\).*/\1/"

tests/test-git-export.out

@@ -40,3 +40,7 b' rename to dst'
  4
  5
 +a
+% nonexistent in tip+chmod
+diff --git a/src b/src
+old mode 100644
+new mode 100755