doc: consolidate --rerere-autoupdate description

The `--rerere-autoupdate` option is shared across 5 commands, and
are described the same way because it works exactly the same way in
these commands.

Create a separate file and include it from the help pages for these
commands, so that we can improve the description at one place to
improve all of them at once, and keep them in sync.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -112,10 +112,7 @@ default.   You can use `--no-utf8` to override this.
 	am.threeWay configuration variable. For more information,
 	see am.threeWay in linkgit:git-config[1].
 
---rerere-autoupdate::
---no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+include::rerere-options.txt[]
 
 --ignore-space-change::
 --ignore-whitespace::

Documentation/git-cherry-pick.txt

@@ -156,10 +156,7 @@ effect to your index in a row.
 	Pass the merge strategy-specific option through to the
 	merge strategy.  See linkgit:git-merge[1] for details.
 
---rerere-autoupdate::
---no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+include::rerere-options.txt[]
 
 SEQUENCER SUBCOMMANDS
 ---------------------

Documentation/git-merge.txt

@@ -90,10 +90,7 @@ invocations. The automated message can include the branch description.
 If `--log` is specified, a shortlog of the commits being merged
 will be appended to the specified message.
 
---rerere-autoupdate::
---no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+include::rerere-options.txt[]
 
 --overwrite-ignore::
 --no-overwrite-ignore::

Documentation/git-rebase.txt

@@ -376,10 +376,7 @@ See also INCOMPATIBLE OPTIONS below.
 +
 See also INCOMPATIBLE OPTIONS below.
 
---rerere-autoupdate::
---no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+include::rerere-options.txt[]
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::

Documentation/git-revert.txt

@@ -112,10 +112,7 @@ effect to your index in a row.
 	Pass the merge strategy-specific option through to the
 	merge strategy.  See linkgit:git-merge[1] for details.
 
---rerere-autoupdate::
---no-rerere-autoupdate::
-	Allow the rerere mechanism to update the index with the
-	result of auto-conflict resolution if possible.
+include::rerere-options.txt[]
 
 --reference::
 	Instead of starting the body of the log message with "This

Documentation/rerere-options.txt

@@ -0,0 +1,4 @@
+--rerere-autoupdate::
+--no-rerere-autoupdate::
+	Allow the rerere mechanism to update the index with the
+	result of auto-conflict resolution if possible.