maps: add the "exists" method to RevMap This is a part of the bigger plan to get rid of reading or writing rev_map directly without going through the RevMap class. Before this patch, there is no easy way to know if the RevMap file exists or not, so the revset functions "fromsvn" and "svnrev" choose to read the rev_map file directly, and throw an exception if the file does not exist. This patch adds the "exists" method that will be used in svnmeta next patch, so these revset functions can test if the file exists without reading the rev_map file directly.