Add support for returning typed value by config.git_config.

This patch allows us to specify a given type to any git config
option, and have git_config config the option value (a string)
into that type, raising an InvalidUpdate error if the conversion
failed.

Currently no such option is taking advantage of this features.
Transition of the relevant options will be done piecemeal.

Part of LC27-007.

hooks/config.py

@@ -1,4 +1,6 @@
+from errors import InvalidUpdate
 from git import git
+from type_conversions import to_type
 
 # A dictionary of all git config names that this module can query.
 #   - The key used for this table is the config name.
@@ -57,8 +59,28 @@ def git_config(option_name):
 
     if __git_config_map is None:
         initialize_git_config_map()
+    val = __git_config_map[option_name]
 
-    return __git_config_map[option_name]
+    # If this option as a 'type' specified, then convert it to
+    # this type if necessary.  We do this here, rather than during
+    # initialize_git_config_map to avoid the potential for causing
+    # an error for options which might not be used in the end.
+    if ('type' in GIT_CONFIG_OPTS[option_name] and isinstance(val, str)):
+        try:
+            val = to_type(val, GIT_CONFIG_OPTS[option_name]['type'])
+        except ValueError:
+            TYPE_NAME_MAP = {bool: 'boolean',
+                             int:  'integer',
+                            }
+            type_name = TYPE_NAME_MAP[GIT_CONFIG_OPTS[option_name]['type']]
+            raise InvalidUpdate(
+                'Invalid %s value: %s (must be %s)'
+                % (option_name, val, type_name))
+        # Save the converted value to avoid having to do it again
+        # the next time we query the same config option.
+        __git_config_map[option_name] = val
+
+    return val
 
 
 def initialize_git_config_map():

hooks/type_conversions.py

@@ -0,0 +1,48 @@
+"""A number of utility functions performing type conversions...
+"""
+
+
+def to_bool(val):
+    """Translate val into a either True or False.
+
+    Raise ValueError if val cannot be converted.
+
+    PARAMETERS
+        val: The value to convert.  It can be either a boolean,
+            in which case this function is a no-op, or a string
+            whose (lowercase) value must then be either 'true'
+            or 'false'.
+
+    REMARKS
+        It would be nice if we could have used the bool builtin
+        function, but this function unfortunately returns something
+        different: It applies the truth test on the argument, and
+        then returns the result of that test.  In other words,
+        bool("False") returns True!
+    """
+    if val in (True, False):
+        return val
+    val_str = str(val).lower()
+    if val_str == 'true':
+        return True
+    if val_str == 'false':
+        return False
+    raise ValueError("invalid boolean value: '%s'", str(val))
+
+
+def to_type(val, new_type):
+    """Translate val to the given type.
+
+    Raise ValueError if val cannot be converted.
+
+    PARAMETERS
+        val: The value to convert.
+        new_type: A callable that indicates the new type for the value.
+            Usually one of the built-in functions such as 'int', or
+            'bool', etc.
+    """
+    # Most of the built-ins perform a translation job, but there
+    # are some exceptions...
+    if new_type == bool:
+        new_type = to_bool
+    return new_type(val)