Correct grouping of parentheses

git_graph_descendant_of was returning the result of an assignment
Added a no path test for git_graph_descendant_of
Merge pull request #2256 from jacquesg/graph-descendant

Correct grouping of parentheses
graph: handle not finding a merge base gracefully

git_merge_base() returns GIT_ENOTFOUND when it cannot find a merge
base. graph_desdendant_of() returns a boolean value (barring any
errors), so it needs to catch the NOTFOUND return value and convert it
into false, as not merge base means it cannot be a descendant.