Make commands to download patch sets configurable

For patch sets on the ChangeScreen different commands for downloading
the patch sets are offered. For some installations not all commands are
needed. Allow Gerrit administrators to configure which download
commands should be offered.

Bug: issue 548
Change-Id: Ida3e00f3b98e37280ecded2e25b20fc0a21abec0
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/config-gerrit.txt

@@ -1012,6 +1012,10 @@ Default is `30 seconds`.
 
 ----
 [download]
+  command = checkout
+  command = cherry_pick
+  command = pull
+  command = format_patch
   scheme = ssh
   scheme = http
   scheme = anon_http
@@ -1021,6 +1025,34 @@ Default is `30 seconds`.
 
 The download section configures the allowed download methods.
 
+[[download.command]]download.command::
++
+Commands that should be offered to download changes.
++
+Multiple commands are supported:
++
+* `checkout`
++
+Command to fetch and checkout the patch set.
++
+* `cherry_pick`
++
+Command to fetch the patch set and to cherry-pick it onto the current
+commit.
++
+* `pull`
++
+Command to pull the patch set.
++
+* `format_patch`
++
+Command to fetch the patch set and to feed it into the `format-patch`
+command.
+
++
+If `download.command` is not specified, all download commands are
+offered.
+
 [[download.scheme]]download.scheme::
 +
 Schemes that should be used to download changes.
@@ -1053,7 +1085,7 @@ generally worked on with the repo multi-repository tool.  This is
 not default, as not all instances will deploy repo.
 
 +
-If download.scheme is not specified, SSH, HTTP and Anonymous HTTP
+If `download.scheme` is not specified, SSH, HTTP and Anonymous HTTP
 downloads are allowed.
 
 [[gerrit]]Section gerrit