patch: man entry for obnam diff

> bzr diff obnam.1.in
=== modified file 'obnam.1.in'
--- obnam.1.in  2012-09-29 13:33:13 +0000
+++ obnam.1.in  2012-10-10 21:58:22 +0000
@@ -140,6 +140,23 @@
 and
 .B \-\-critical-age
 options.
+.IP \(bu
+.B diff
+shows a diff between two generations. If
+.B GENERATION1
+is missing, the generation before
+.B GENERATION2
+is implied, so that
+.B obnam diff GENERATION2
+shows what got changed in
+.B "GENERATION2".
+Changed files are listed one pr. line with a character:
+.B +
+for added files,
+.B *
+for changed files,
+.B -
+and for removed files. Then a single space, and then the file name.
 .SS "Making backups"
 When you run a backup,
 .B obnam