Remove begin/end spec as part of the common prefix

When we define our own --markers='x-begin x-middle x-end', and our code contains as a first
character `x`, then that character will be considered part of the common prefix
and be removed from the code. Here's an example.

    // cog-begin
    // cog.outl('hello world')
    // cog-middle
    // cog-end

The common prefix for this code is `// cog`, which makes the code generator fail
because python will receive the code `.outl('hello world')`, which makes no
sense.

This issue is especially cumbersome to work with if you're invoking a module,
for instance:

    // cog-begin
    // codegen.generate_some_code()
    // cog-middle
    // cog-end

Here the python interpeter will receive `degen.generate_some_code()`.

This patch removes the begin and end specs from the marker line to avoid this
issue.

Author: kstrafe

cogapp/cogapp.py

@@ -125,6 +125,10 @@ def get_code(self):
         # If the markers and lines all have the same prefix
         # (end-of-line comment chars, for example),
         # then remove it from all the lines.
+        for idx, marker in enumerate(self.markers):
+            self.markers[idx] = marker.replace(self.options.begin_spec, "").replace(
+                self.options.end_spec, ""
+            )
         pref_in = common_prefix(self.markers + self.lines)
         if pref_in:
             self.markers = [line.replace(pref_in, "", 1) for line in self.markers]

cogapp/test_cogapp.py

@@ -254,6 +254,30 @@ def test_prefixed_indented_code(self):
         infile = reindent_block(infile)
         self.assertEqual(Cog().process_string(infile), infile)
 
+    def test_markers_overlapping_prefix(self):
+        infile = """\
+            // cog-begin
+            // cog.outl('lorem ipsum')
+            // cog-middle
+            // cog-end
+            """
+
+        outfile = """\
+            // cog-begin
+            // cog.outl('lorem ipsum')
+            // cog-middle
+            lorem ipsum
+            // cog-end
+            """
+
+        infile = reindent_block(infile)
+        outfile = reindent_block(outfile)
+        cog = Cog()
+        cog.options.begin_spec = "cog-begin"
+        cog.options.end_spec = "cog-middle"
+        cog.options.end_output = "cog-end"
+        self.assertEqual(cog.process_string(infile), outfile)
+
     def test_bogus_prefix_match(self):
         infile = """\
             prologue