configure: Enable werror for git worktrees

The configure script checks multiple times whether it works in a git
repository and it does this by "test -e "${source_path}/.git" in 4 cases
but in one case where it tries to enable werror "-d" is used there which
fails on git worktrees as .git is a file then and not a directory.

This changes the test to "-e" as other occurrences.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20190228043503.68494-1-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1836,7 +1836,7 @@ fi
 # Consult white-list to determine whether to enable werror
 # by default.  Only enable by default for git builds
 if test -z "$werror" ; then
-    if test -d "$source_path/.git" && \
+    if test -e "$source_path/.git" && \
         { test "$linux" = "yes" || test "$mingw32" = "yes"; }; then
         werror="yes"
     else