ALGORITHM CHANGE: only diff tree with first parent

Running git deps on FOO^! is effectively answering the question "which
commits would I need in order to be able to cleanly cherry-pick commit
FOO?"  Drilling down further, that could be rephrased more precisely
as "which commits would I need in my current branch in order to be
able to cleanly apply the diff which commit FOO applies to its
parent?"

However, in the case where FOO is a merge commit with multiple
parents, typically the first parent P1 is the parent which is
contained by the merge's target branch B1.  That means that the merge
commit FOO has the effect of applying the diff between P1's tree and
the FOO's tree to P1.  This could be expressed as:

    tree(P1) + diff(tree(P1), tree(FOO)) == tree(FOO)

Therefore the question git-deps needs to answer when operating on a
commit with multiple parents is "which commits would I need in my
current branch in order to be able to cleanly apply diff(tree(P1),
tree(FOO)) to it?"

However, the current algorithm runs the blame analysis not only on
diff(tree(P1), tree(FOO)), but on diff(tree(Px), tree(FOO)) for
*every* parent.  This is problematic, because for instance if the
target branch contains commits which are not on P2's branch, then

    diff(tree(P2), tree(FOO))

will regress any changes provided by those commits.  This will
introduce extra dependencies which incorrectly answer the above
question we are trying to answer.

Therefore change the algorithm to only diff against the first parent.

This is very similar in nature to the -m option of git cherry-pick:

    https://stackoverflow.com/questions/12626754/git-cherry-pick-syntax-and-merge-branches/12628579#12628579

In the future it may be desirable to add an analogous -m option to
git-deps.

Sem-Ver: api-break

1 files changed, 2 insertions, 2 deletions

diff --git a/git_deps/detector.py b/git_deps/detector.py
index 8e977cc..d358f12 100644
--- a/git_deps/detector.py
+++ b/git_deps/detector.py
@@ -118,8 +118,8 @@ class DependencyDetector(object):
 
             self.notify_listeners('new_commit', dependent)
 
-            for parent in dependent.parents:
-                self.find_dependencies_with_parent(dependent, parent)
+            parent = dependent.parents[0]
+            self.find_dependencies_with_parent(dependent, parent)
             self.done.append(dependent_sha1)
             self.done_d[dependent_sha1] = True
             self.logger.info("  Found all dependencies for %s" %