patch: ensure valid git diffs if source/destination file is missing (issue4046)

This is arguably a workaround, a better fix may be in the repo to
ensure that it won't list a file 'modified' unless there is a file
context for the previous version.

mercurial/patch.py

@@ -1703,7 +1703,7 b' def trydiff(repo, revs, ctx1, ctx2, modi'
             tn = getfilectx(f, ctx2).data()
         a, b = f, f
         if opts.git or losedatafn:
-            if f in added:
+            if f in added or (f in modified and to is None):
                 mode = gitmode[ctx2.flags(f)]
                 if f in copy or f in copyto:
                     if opts.git:
@@ -1739,7 +1739,7 b' def trydiff(repo, revs, ctx1, ctx2, modi'
                 if not opts.git and not tn:
                     # regular diffs cannot represent new empty file
                     losedatafn(f)
-            elif f in removed:
+            elif f in removed or (f in modified and tn is None):
                 if opts.git:
                     # have we already reported a copy above?
                     if ((f in copy and copy[f] in added

tests/test-git-export.t

@@ -360,3 +360,23 b' There should be a trailing TAB if there '
   +foo
   $ hg ci -m 'add filename with spaces'
 
+Additions should be properly marked even in the middle of a merge
+
+  $ hg up -r -2
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo "New File" >> inmerge
+  $ hg add inmerge
+  $ hg ci -m "file in merge"
+  created new head
+  $ hg up 23
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg diff -g
+  diff --git a/inmerge b/inmerge
+  new file mode 100644
+  --- /dev/null
+  +++ b/inmerge
+  @@ -0,0 +1,1 @@
+  +New File

tests/test-shelve.t

@@ -228,6 +228,7 b' ensure that we have a merge with unresol'
   +a
   +>>>>>>> other
   diff --git a/b.rename/b b/b.rename/b
+  new file mode 100644
   --- /dev/null
   +++ b/b.rename/b
   @@ -0,0 +1,1 @@
@@ -239,6 +240,7 b' ensure that we have a merge with unresol'
   @@ -1,1 +0,0 @@
   -b
   diff --git a/c.copy b/c.copy
+  new file mode 100644
   --- /dev/null
   +++ b/c.copy
   @@ -0,0 +1,1 @@