win32: emulate Git for Windows in symlink support

Emulate the Git for Windows `core.symlinks` support.  Since symbolic
links are generally enabled for Administrator (and _may_ be enabled due
to enabling Developer mode) but symbolic links are still sufficiently
uncommon on Windows that Git users are expected to explicitly opt-in to
symbolic links by enabling `core.symlinks=true` in a global (or xdg or
system) configuration.

When `core.symlinks=true` is set globally _and_ symbolic links support
is detected then new repositories created will not have a
`core.symlinks` set.  If `core.symlinks` is _not_ set then no detection
will be performed, and `core.symlinks=false` will be set in the
repository configuration.

src/repository.c

@@ -1411,24 +1411,56 @@ static bool is_filesystem_case_insensitive(const char *gitdir_path)
 
 static bool are_symlinks_supported(const char *wd_path)
 {
+	git_config *config = NULL;
 	git_buf path = GIT_BUF_INIT;
 	int fd;
 	struct stat st;
-	int symlinks_supported = -1;
+	bool symlinks = false;
+
+	/*
+	 * To emulate Git for Windows, symlinks on Windows must be explicitly
+	 * opted-in.  We examine the system configuration for a core.symlinks
+	 * set to true.  If found, we then examine the filesystem to see if
+	 * symlinks are _actually_ supported by the current user.  If that is
+	 * _not_ set, then we do not test or enable symlink support.
+	 */
+#ifdef GIT_WIN32
+	git_buf global_buf = GIT_BUF_INIT;
+	git_buf xdg_buf = GIT_BUF_INIT;
+	git_buf system_buf = GIT_BUF_INIT;
+	git_buf programdata_buf = GIT_BUF_INIT;
+
+	git_config_find_global(&global_buf);
+	git_config_find_xdg(&xdg_buf);
+	git_config_find_system(&system_buf);
+	git_config_find_programdata(&programdata_buf);
+
+	if (load_config(&config, NULL,
+	    path_unless_empty(&global_buf),
+	    path_unless_empty(&xdg_buf),
+	    path_unless_empty(&system_buf),
+	    path_unless_empty(&programdata_buf)) < 0)
+		goto done;
+
+	if (git_config_get_bool(&symlinks, config, "core.symlinks") < 0 || !symlinks)
+		goto done;
+#endif
 
 	if ((fd = git_futils_mktmp(&path, wd_path, 0666)) < 0 ||
-		p_close(fd) < 0 ||
-		p_unlink(path.ptr) < 0 ||
-		p_symlink("testing", path.ptr) < 0 ||
-		p_lstat(path.ptr, &st) < 0)
-		symlinks_supported = false;
-	else
-		symlinks_supported = (S_ISLNK(st.st_mode) != 0);
+	    p_close(fd) < 0 ||
+	    p_unlink(path.ptr) < 0 ||
+	    p_symlink("testing", path.ptr) < 0 ||
+	    p_lstat(path.ptr, &st) < 0)
+		goto done;
+
+	symlinks = (S_ISLNK(st.st_mode) != 0);
 
 	(void)p_unlink(path.ptr);
-	git_buf_dispose(&path);
 
-	return symlinks_supported;
+done:
+	git_buf_dispose(&path);
+	git_config_free(config);
+	return symlinks;
 }
 
 static int create_empty_file(const char *path, mode_t mode)