[gnome-apps-nightly/wip/mjog/update-geary-branch] Update Geary branch name

[Michael Gratton <mjog src gnome org>]

commit a12aa593c8ca95a0cc9f7b66da0d7f525a29882a
Author: Michael James Gratton <mike vee net>
Date:   Tue Apr 2 14:19:45 2019 +1100

    Update Geary branch name
    
    See GNOME/geary#324

 org.gnome.Geary.app | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/org.gnome.Geary.app b/org.gnome.Geary.app
index de9193e..5d53d27 100644
--- a/org.gnome.Geary.app
+++ b/org.gnome.Geary.app
@@ -1,3 +1,4 @@
 ID=org.gnome.Geary
 JSON=org.gnome.Geary.json
 GITURL=https://gitlab.gnome.org/GNOME/geary.git
+GITBRANCH=mainline