build: Make git log invocation immune to local configuration

Some of the options that can be configured globally can leak local
repository information when generating the ChangeLog, such as the
--decorate=long option for example.

Makefile.am

@@ -181,6 +181,7 @@ dist-hook:
 				exit 1 ; \
 			fi ; \
 		done ; \
+		XDG_CONFIG_HOME= HOME= \
 		git log -C --stat 1.15.0.. >$(distdir)/ChangeLog; \
 	fi
 

debian/changelog

@@ -76,6 +76,7 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     - Split libps and libkvm detection into their own macros and variables.
     - Make it possible to build without system libmd.
     - Add a configuration summary to configure output.
+    - Make git log invocation immune to local configuration.
   * Test suite:
     - Add a unit test to compile perl code with warnings.
     - Add a unit test for the trigger deferred parser.