Handle RST files

Do not build all manuals if only RST files are changed.

Change-Id: Ie0790d7bebd676c9430a4295fe4421cbd74cbd8c

RELEASE_NOTES.rst

@@ -1,6 +1,12 @@
 Release notes
 =============
 
+0.23
+----
+
+* ``openstack-doc-test``: Don't build all books if only RST files are
+  changed.
+
 0.22
 ----
 

os_doc_tools/doctest.py

@@ -376,6 +376,27 @@ def only_po_touched():
     return locale_changed and not other_changed
 
 
+def only_rst_touched():
+    """Check whether only RST files are touched."""
+
+    try:
+        git_args = ["git", "diff", "--name-only", "HEAD~1", "HEAD"]
+        modified_files = check_output(git_args).strip().split()
+    except (subprocess.CalledProcessError, OSError) as e:
+        print("git failed: %s" % e)
+        sys.exit(1)
+
+    rst_changed = False
+    other_changed = False
+    for f in modified_files:
+        if f.endswith(".rst"):
+            rst_changed = True
+        else:
+            other_changed = True
+
+    return rst_changed and not other_changed
+
+
 def check_modified_affects_all(rootdir):
     """Check whether special files were modified.
 
@@ -1571,6 +1592,10 @@ def doctest():
             and only_po_touched()):
         print("Only files in locale directories changed, nothing to do.\n")
         return
+    # If only RST files are touched, there's nothing to do.
+    if (not CONF.force and only_rst_touched()):
+        print("Only RST files changed, nothing to do.\n")
+        return
 
     if CONF.check_syntax or CONF.check_niceness or CONF.check_links:
         if CONF.force: