Disallow '\' in ref names

This is asking for trouble since '\' is a directory separator in
Windows and thus may produce unpredictable results.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -38,6 +38,8 @@ imposes the following rules on how references are named:
 
 . They cannot contain a sequence `@{`.
 
+- They cannot contain a `\\`.
+
 These rules make it easy for shell script based tools to parse
 reference names, pathname expansion by the shell when a reference name is used
 unquoted (by mistake), and also avoids ambiguities in certain

refs.c

@@ -682,12 +682,13 @@ int for_each_rawref(each_ref_fn fn, void *cb_data)
  * - it has ASCII control character, "~", "^", ":" or SP, anywhere, or
  * - it ends with a "/".
  * - it ends with ".lock"
+ * - it contains a "\" (backslash)
  */
 
 static inline int bad_ref_char(int ch)
 {
 	if (((unsigned) ch) <= ' ' ||
-	    ch == '~' || ch == '^' || ch == ':')
+	    ch == '~' || ch == '^' || ch == ':' || ch == '\\')
 		return 1;
 	/* 2.13 Pattern Matching Notation */
 	if (ch == '?' || ch == '[') /* Unsupported */