man: Document environment variables used by source format modules

debian/changelog

@@ -6,6 +6,8 @@ dpkg (1.18.12) UNRELEASED; urgency=medium
     - Rename FILE FORMATS section to FILES in dpkg-source(1).
     - Use "dpkg suite" as the manual item in man pages .TH macro.
     - Use %VERSION% in dpkg-statoverride(1) source item in man page .TH macro.
+    - Document environment variables used by source format modules in
+      dpkg-source(1).
 
  -- Guillem Jover <guillem@debian.org>  Sun, 06 Nov 2016 05:03:23 +0100
 

man/dpkg-source.man

@@ -777,6 +777,22 @@ of that fact.
 .B SOURCE_DATE_EPOCH
 If set, it will be used as the timestamp (as seconds since the epoch) to
 clamp the mtime in the \fBtar\fP(5) file entries.
+.TP
+.B VISUAL
+.TQ
+.B EDITOR
+Used by the “2.0” and “3.0 (quilt)” source format modules.
+.TP
+.B GIT_DIR
+.TQ
+.B GIT_INDEX_FILE
+.TQ
+.B GIT_OBJECT_DIRECTORY
+.TQ
+.B GIT_ALTERNATE_OBJECT_DIRECTORIES
+.TQ
+.B GIT_WORK_TREE
+Used by the “3.0 (git)” source format modules.
 .
 .SH FILES
 .SS debian/source/format