autoconf: clean temporary file mak.append

When configure is interrupted in the middle it leaves
config.mak.append behind.  Add it to .gitignore and make sure
$(MAKE) clean removes it.

Also earlier .gitignore listed config.mak.in which is a tracked
file.  Fix it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

.gitignore

@@ -140,7 +140,7 @@ config.mak
 autom4te.cache
 config.log
 config.status
-config.mak.in
 config.mak.autogen
+config.mak.append
 configure
 git-blame

Makefile

@@ -793,7 +793,7 @@ clean:
 	rm -f $(ALL_PROGRAMS) $(BUILT_INS) git$X
 	rm -f *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h TAGS tags
 	rm -rf autom4te.cache
-	rm -f config.log config.mak.autogen configure config.status config.cache
+	rm -f configure config.log config.mak.autogen config.mak.append config.status config.cache
 	rm -rf $(GIT_TARNAME) .doc-tmp-dir
 	rm -f $(GIT_TARNAME).tar.gz git-core_$(GIT_VERSION)-*.tar.gz
 	rm -f $(htmldocs).tar.gz $(manpages).tar.gz