Index: subversion/include/svn_mergeinfo.h
===================================================================
--- subversion/include/svn_mergeinfo.h	(revision 889271)
+++ subversion/include/svn_mergeinfo.h	(working copy)
@@ -157,6 +157,9 @@
  * inheritability are also allowed, but will be combined into a single
  * range when placed into @a *mergeinfo.
  *
+ * @a input may contain relative merge source paths, but these are
+ * converted to absolute paths in @a *mergeinfo.
+ *
  * @since New in 1.5.
  */
 svn_error_t *
Index: subversion/libsvn_subr/mergeinfo.c
===================================================================
--- subversion/libsvn_subr/mergeinfo.c	(revision 889271)
+++ subversion/libsvn_subr/mergeinfo.c	(working copy)
@@ -73,8 +73,10 @@
 
 /* pathname -> PATHNAME */
 static svn_error_t *
-parse_pathname(const char **input, const char *end,
-               svn_stringbuf_t **pathname, apr_pool_t *pool)
+parse_pathname(const char **input,
+               const char *end,
+               svn_stringbuf_t **pathname,
+               apr_pool_t *pool)
 {
   const char *curr = *input;
   const char *last_colon = NULL;
@@ -96,7 +98,20 @@
     return svn_error_create(SVN_ERR_MERGEINFO_PARSE_ERROR, NULL,
                             _("No pathname preceding ':'"));
 
-  *pathname = svn_stringbuf_ncreate(*input, last_colon - *input, pool);
+  /* Tolerate relative repository paths, but convert them to absolute. */
+  if (**input == '/')
+    {
+      *pathname = svn_stringbuf_ncreate(*input, last_colon - *input, pool);
+    }
+  else
+    {
+      const char *repos_rel_path = apr_pstrndup(pool, *input,
+                                                last_colon - *input);
+      *pathname = svn_stringbuf_ncreate(apr_psprintf(pool, "/%s",
+                                        repos_rel_path),
+                                        (last_colon - *input) + 1, pool);
+    }
+
   *input = last_colon;
 
   return SVN_NO_ERROR;