[sancov] Fix broken links and displaced coloring in coverage-report-server.py

This patch fixes two issues:

* Fixed relative links to source files
* Enumeration of lines in source files starts from 1 instead of 0 to
  align with .symcov files generated by sancov -symbolize

Patch by Dmitiriy Nikiforov.

Differential Revision: https://reviews.llvm.org/D31038

llvm-svn: 298250