flags: introduce explicit testing for merging change to exec flag

It turns out that we do not seems to test the simple case for merging exec flag
changes. More advanced case are test (merging exec flag without a common
ancestors, merging with a symlink, etc…) but not the basic.

We are about introduce various fixes to merging flag change across renames,
having  the most basic case tested first seems useful.

note: We are only testing "adding" an exec flag here, not removing it. We
introduce basic test on stable and will consolidate them on default.

Differential Revision: https://phab.mercurial-scm.org/D8529

tests/test-merge-exec.t

@@ -0,0 +1,82 b''
+===============================================
+Testing merge involving change to the exec flag
+===============================================
+
+#require execbit
+
+
+Initial setup
+==============
+
+
+  $ hg init base-repo
+  $ cd base-repo
+  $ cat << EOF > a
+  > 1
+  > 2
+  > 3
+  > 4
+  > 5
+  > 6
+  > 7
+  > 8
+  > 9
+  > EOF
+  $ touch b
+  $ hg add a b
+  $ hg commit -m "initial commit"
+  $ cd ..
+
+Testing merging mode change
+===========================
+
+setup
+
+Change on one side, executable bit on the other
+
+  $ hg clone base-repo simple-merge-repo
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd simple-merge-repo
+  $ chmod +x a
+  $ hg ci -m "make a executable, no change"
+  $ [ -x a ] || echo "executable bit not recorded"
+  $ hg up ".^"
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat << EOF > a
+  > 1
+  > 2
+  > 3
+  > 4
+  > 5
+  > 6
+  > 7
+  > x
+  > 9
+  > EOF
+  $ hg commit -m "edit end of file"
+  created new head
+
+merge them (from the update side)
+
+  $ hg merge 'desc("make a executable, no change")'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg st
+  M a
+  $ [ -x a ] || echo "executable bit lost"
+
+merge them (from the chmod side)
+
+  $ hg up -C 'desc("make a executable, no change")'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge 'desc("edit end of file")'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg st
+  M a
+  $ [ -x a ] || echo "executable bit lost"
+
+
+  $ cd ..
+