Documentation: include 'merge.branchdesc' for merge and config as well

'merge.branchdesc' is only mentioned in the docs of 'git fmt-merge-msg'.

The description of 'merge.log' is already duplicated between
'merge-config.txt' and 'git-fmt-merge-msg.txt'; instead of duplicating the
description of another config variable, extract the descriptions of both
of these variables from 'git-fmt-merge-msg.txt' into a separate file and
include it there and in 'merge-config.txt'.

Leave 'merge.summary' only in git-fmt-merge-msg.txt, as it is marked
as deprecated.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fmt-merge-msg-config.txt

@@ -0,0 +1,10 @@
+merge.branchdesc::
+	In addition to branch names, populate the log message with
+	the branch description text associated with them.  Defaults
+	to false.
+
+merge.log::
+	In addition to branch names, populate the log message with at
+	most the specified number of one-line descriptions from the
+	actual commits that are being merged.  Defaults to false, and
+	true is a synonym for 20.

Documentation/git-fmt-merge-msg.txt

@@ -51,17 +51,7 @@ OPTIONS
 
 CONFIGURATION
 -------------
-
-merge.branchdesc::
-	In addition to branch names, populate the log message with
-	the branch description text associated with them.  Defaults
-	to false.
-
-merge.log::
-	In addition to branch names, populate the log message with at
-	most the specified number of one-line descriptions from the
-	actual commits that are being merged.  Defaults to false, and
-	true is a synonym for 20.
+include::fmt-merge-msg-config.txt[]
 
 merge.summary::
 	Synonym to `merge.log`; this is deprecated and will be removed in

Documentation/merge-config.txt

@@ -26,11 +26,7 @@ merge.ff::
 	allowed (equivalent to giving the `--ff-only` option from the
 	command line).
 
-merge.log::
-	In addition to branch names, populate the log message with at
-	most the specified number of one-line descriptions from the
-	actual commits that are being merged.  Defaults to false, and
-	true is a synonym for 20.
+include::fmt-merge-msg-config.txt[]
 
 merge.renameLimit::
 	The number of files to consider when performing rename detection