templater: try to read %include in mapfiles from resources

The "show" style is an example of a style that uses an "%include"
statement in its definition. This patch makes `hg log --style show`
work.

This takes the number of failing tests with PyOxidizer from 72 to 62.

Differential Revision: https://phab.mercurial-scm.org/D8896

mercurial/templater.py

@@ -835,11 +835,17 b' def _readmapfile(fp, mapfile):'
             if os.path.isfile(abs):
                 subresource = util.posixfile(abs, b'rb')
         if not subresource:
-            dir = templatedir()
-            if dir:
-                abs = os.path.normpath(os.path.join(dir, rel))
-                if os.path.isfile(abs):
-                    subresource = util.posixfile(abs, b'rb')
+            if pycompat.ossep not in rel:
+                abs = rel
+                subresource = resourceutil.open_resource(
+                    b'mercurial.templates', rel
+                )
+            else:
+                dir = templatedir()
+                if dir:
+                    abs = os.path.normpath(os.path.join(dir, rel))
+                    if os.path.isfile(abs):
+                        subresource = util.posixfile(abs, b'rb')
         if subresource:
             data = subresource.read()
             conf.parse(