Disallow refs starting with a non-letter or digit

Change the regexp for validating refs to require at least one letter or digit
before allowing the other special chars in the set `[.-_]`. Names that start
with `.` are traditionally Unix hidden files; let's ignore them under the
assumption they're metadata for some other tool, and we don't want to
potentially conflict with the special `.` and `..` Unix directory entries.
Further, names starting with `-` are problematic for Unix cmdline option
processing; there's no good reason to support that. Finally, disallow `_` just
on general principle - it's simpler to say that ref identifiers must start with
a letter or digit.

We also ignore any existing files (that might be previously created refs) that
start with `.` in the `refs/` directory - there's a Red Hat tool for content
management that injects `.rsync` files, which is why this patch was first
written.

V1: Update to ban all refs starting with a non-letter/digit, and
    also add another call to `ostree_validate_rev` in the pull
    code.

Closes: https://github.com/ostreedev/ostree/issues/1285

Closes: #1286
Approved by: jlebon

src/libostree/ostree-core.c

@@ -142,7 +142,10 @@ ostree_validate_checksum_string (const char *sha256,
   return ostree_validate_structureof_checksum_string (sha256, error);
 }
 
-#define OSTREE_REF_FRAGMENT_REGEXP "[-._\\w\\d]+"
+/* This used to allow leading - and ., but was changed in
+ * https://github.com/ostreedev/ostree/pull/1286
+ */
+#define OSTREE_REF_FRAGMENT_REGEXP "[\\w\\d][-._\\w\\d]*"
 #define OSTREE_REF_REGEXP "(?:" OSTREE_REF_FRAGMENT_REGEXP "/)*" OSTREE_REF_FRAGMENT_REGEXP
 #define OSTREE_REMOTE_NAME_REGEXP OSTREE_REF_FRAGMENT_REGEXP
 
@@ -196,6 +199,26 @@ ostree_parse_refspec (const char   *refspec,
   return TRUE;
 }
 
+gboolean
+_ostree_validate_ref_fragment (const char *fragment,
+                               GError    **error)
+{
+  static GRegex *regex;
+  static gsize regex_initialized;
+  if (g_once_init_enter (&regex_initialized))
+    {
+      regex = g_regex_new ("^" OSTREE_REF_FRAGMENT_REGEXP "$", 0, 0, NULL);
+      g_assert (regex);
+      g_once_init_leave (&regex_initialized, 1);
+    }
+
+  g_autoptr(GMatchInfo) match = NULL;
+  if (!g_regex_match (regex, fragment, 0, &match))
+    return glnx_throw (error, "Invalid ref fragment '%s'", fragment);
+
+  return TRUE;
+}
+
 /**
  * ostree_validate_rev:
  * @rev: A revision string