extdiff: copy back execbit-only changes to the working directory

Some tools like BeyondCompare allow the file mode to be changed.  The change
was previously applied if the content of the file changed (either according to
size or mtime), but was not being copied back for a mode-only change.  That
would seem to indicate handling this in an 'elif' branch, but I opted not to in
order to avoid copying back the mode without the content changes when mtime and
size are unchanged.  (Yes, that's a rare corner case, but all the more reason
not to have a subtle difference in behavior.)

The only way I can think to handle this undetected change is to set each file in
the non-wdir() snapshot to readonly, and check for that attribute (as well as
mtime) when deciding to copy back.  That would avoid the overhead of copying the
whole file when only the mode changed.  But a chmod in a diff tool is likely
rare.  See also affd753ddaf1.

hgext/extdiff.py

@@ -280,7 +280,11 b' def dodiff(ui, repo, cmdline, pats, opts'
             # all changes.  A size check will detect more cases, but not all.
             # The only certain way to detect every case is to diff all files,
             # which could be expensive.
-            if cpstat.st_mtime != st.st_mtime or cpstat.st_size != st.st_size:
+            # copyfile() carries over the permission, so the mode check could
+            # be in an 'elif' branch, but for the case where the file has
+            # changed without affecting mtime or size.
+            if (cpstat.st_mtime != st.st_mtime or cpstat.st_size != st.st_size
+                or (cpstat.st_mode & 0o100) != (st.st_mode & 0o100)):
                 ui.debug('file changed while diffing. '
                          'Overwriting: %s (src: %s)\n' % (working_fn, copy_fn))
                 util.copyfile(copy_fn, working_fn)

tests/test-extdiff.t

@@ -329,6 +329,7 b' Fallback to merge-tools.tool.executable|'
   > # Mimic a tool that syncs all attrs, including mtime
   > cp $1/a $2/a
   > touch -r $1/a $2/a
+  > chmod +x $2/a
   > echo "** custom diff **"
   > EOF
 #if execbit
@@ -366,6 +367,32 b' of $tool (and fail).'
   $ cat a
   a
 
+#if execbit
+  $ [ -x a ]
+
+  $ cat > 'dir/tool.sh' << 'EOF'
+  > #!/bin/sh
+  > chmod -x $2/a
+  > echo "** custom diff **"
+  > EOF
+
+  $ hg --debug tl --config extdiff.tl= --config merge-tools.tl.executable=$tool
+  making snapshot of 2 files from rev * (glob)
+    a
+    b
+  making snapshot of 2 files from working directory
+    a
+    b
+  running '$TESTTMP/a/dir/tool.sh a.* a' in */extdiff.* (glob)
+  ** custom diff **
+  file changed while diffing. Overwriting: $TESTTMP/a/a (src: */extdiff.*/a/a) (glob)
+  cleaning up temp directory
+  [1]
+
+  $ [ -x a ]
+  [1]
+#endif
+
   $ cd ..
 
 #if symlink