[gitg] man: Document GIT_DIR env

[Alberto Fanjul <albfan src gnome org>]

commit c2b86910630fe212583a6a703c5762eeee98f1e7
Author: Alberto Fanjul <albertofanjul gmail com>
Date:   Thu Dec 27 12:43:07 2018 +0100

    man: Document GIT_DIR env

 data/gitg.1 | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/data/gitg.1 b/data/gitg.1
index 64774acb..54e22e66 100644
--- a/data/gitg.1
+++ b/data/gitg.1
@@ -40,6 +40,11 @@ which has the same format as used with
 .B gitg
 \-\-left\-right HEAD..FETCH_HEAD)
 
+.SH ENVIRONMENT VARIABLES
+
+.B GIT_DIR
+If the GIT_DIR environment variable is set it specifies a path to use instead of the default .git
+
 .SH AUTHORS
 Jesse van den Kieboom  <jesse icecrew nl>
 Alberto Fanjul  <albfan gnome org>