[FIX] runbot_merge: extraction of authorship info during rebase

Turns out I've always been mistaken about the handling of quotes
*inside* shell parameters, apparently they are always consumed by the
shell unless nested so

    --foo="bar"

reaches the underlying program as

    --foo=bar

This means when using subprocess (without shell=True), adding the
quotes leads to mishandling of the parameters (as the subprocess now
has quotes it's not equipped to deal with).

This exact error is made in the `--pretty` parameter of git show,
locally this results in the author name and the committer email being
terminated by double quotes although somehow other layers seem to
exclude those from the end result (I assume `commit-tree` strips the
quotes from the envvars under the assumption that users can mistakenly
quote them or something?

Anyway while it does not seem harmful (so far), better safe than
sorry.

runbot_merge/git.py

@@ -160,7 +160,7 @@ class Repo:
 
         mapping = {}
         for original, tree in zip(commits, new_trees):
-            authorship = check(repo.show('--no-patch', '--pretty="%an%n%ae%n%ai%n%cn%n%ce"', original['sha']))
+            authorship = check(repo.show('--no-patch', '--pretty=%an%n%ae%n%ai%n%cn%n%ce', original['sha']))
             author_name, author_email, author_date, committer_name, committer_email =\
                 authorship.stdout.splitlines()