doc: fix make distcheck with git worktrees

In a git worktree, .git is a regular (text) file, not a directory.

Signed-off-by: Thomas Guillem <thomas@gllm.fr>

doc/Makefile.am

@@ -75,7 +75,7 @@ doc: Doxyfile
 	doxygen
 
 $(CHANGELOGS): Makefile.am
-	if test -d "$(top_srcdir)/.git"; then \
+	if test -e "$(top_srcdir)/.git"; then \
 		y="$@"; y="$${y##ChangeLog-}" ; \
 		git --git-dir="$(top_srcdir)/.git" log \
 			--since="$$y-01-01" \