annotate: calculate line count correctly Before this patch, the "lines" function inside "annotate" returns 1 for empty text (''). This patch makes it 0. Because the function should match mdiff.splitnewlines (used by mdiff.allblocks), or s.splitlines (used at the end of the "annotate" method). Both len(mdiff.splitnewlines('')) and len(''.splitlines(True)) are 0. This issue was discovered while testing fastannotate [1]. I could not find a test case to reveal this issue. However in theory this could reduce memory usage a little bit, and avoids surprises when people are touching this area in the future. [1]: https://bitbucket.org/facebook/hg-experimental/commits/525b3b98e93a