splicemap: improve error handling when source is hg (
issue2084)
1. Introduced 2 levels of error handling for splicemap files
a. Check the splicemap file for rules which are same across different
types of source repos. This is done through enhancing parsesplicemap
function
b. Check revision string formats. Each repo may have their own format.
This is done usign checkrevformat function
c. Implemented the above two for hg