ExternalProject: Add option to set `git clone -o` argument

Add a `GIT_REMOTE_NAME` option to `ExternalProject_Add` to support

  git clone --origin <name>

Default to `origin` if not specified.

Help/release/dev/ExternalProject-git-clone-o.rst

+ExternalProject-git-clone-o
+---------------------------
+
+* The :module:`ExternalProject` module learned a new ``GIT_REMOTE_NAME``
+  option to control the ``git clone --origin`` value.

Modules/ExternalProject.cmake

@@ -57,6 +57,8 @@ Create custom targets to build projects in external trees
     URL of git repo
   ``GIT_TAG <tag>``
     Git branch name, commit id or tag
+  ``GIT_REMOTE_NAME <name>``
+    The optional name of the remote, default to ``origin``
   ``GIT_SUBMODULES <module>...``
     Git submodules that shall be updated, all if empty
   ``HG_REPOSITORY <url>``
@@ -494,7 +496,7 @@ define_property(DIRECTORY PROPERTY "EP_UPDATE_DISCONNECTED" INHERITED
   "ExternalProject module."
   )
 
-function(_ep_write_gitclone_script script_filename source_dir git_EXECUTABLE git_repository git_tag git_submodules src_name work_dir gitclone_infofile gitclone_stampfile)
+function(_ep_write_gitclone_script script_filename source_dir git_EXECUTABLE git_repository git_tag git_remote_name git_submodules src_name work_dir gitclone_infofile gitclone_stampfile)
   file(WRITE ${script_filename}
 "if(\"${git_tag}\" STREQUAL \"\")
   message(FATAL_ERROR \"Tag for git checkout should not be empty.\")
@@ -524,7 +526,7 @@ set(error_code 1)
 set(number_of_tries 0)
 while(error_code AND number_of_tries LESS 3)
   execute_process(
-    COMMAND \"${git_EXECUTABLE}\" clone \"${git_repository}\" \"${src_name}\"
+    COMMAND \"${git_EXECUTABLE}\" clone --origin \"${git_remote_name}\" \"${git_repository}\" \"${src_name}\"
     WORKING_DIRECTORY \"${work_dir}\"
     RESULT_VARIABLE error_code
     )
@@ -645,7 +647,7 @@ endif()
 endfunction()
 
 
-function(_ep_write_gitupdate_script script_filename git_EXECUTABLE git_tag git_submodules git_repository work_dir)
+function(_ep_write_gitupdate_script script_filename git_EXECUTABLE git_tag git_remote_name git_submodules git_repository work_dir)
   if(NOT GIT_VERSION_STRING VERSION_LESS 1.7.6)
     set(git_stash_save_options --all --quiet)
   else()
@@ -687,7 +689,7 @@ if(\"\${show_ref_output}\" MATCHES \"refs/remotes/${git_tag}\")
   set(git_remote \"\${CMAKE_MATCH_1}\")
   set(git_tag \"\${CMAKE_MATCH_2}\")
 else()
-  set(git_remote \"origin\")
+  set(git_remote \"${git_remote_name}\")
   set(git_tag \"${git_tag}\")
 endif()
 
@@ -1749,6 +1751,11 @@ function(_ep_add_download_command name)
     endif()
     get_property(git_submodules TARGET ${name} PROPERTY _EP_GIT_SUBMODULES)
 
+    get_property(git_remote_name TARGET ${name} PROPERTY _EP_GIT_REMOTE_NAME)
+    if(NOT git_remote_name)
+      set(git_remote_name "origin")
+    endif()
+
     # For the download step, and the git clone operation, only the repository
     # should be recorded in a configured RepositoryInfo file. If the repo
     # changes, the clone script should be run again. But if only the tag
@@ -1772,7 +1779,7 @@ function(_ep_add_download_command name)
     # The script will delete the source directory and then call git clone.
     #
     _ep_write_gitclone_script(${tmp_dir}/${name}-gitclone.cmake ${source_dir}
-      ${GIT_EXECUTABLE} ${git_repository} ${git_tag} "${git_submodules}" ${src_name} ${work_dir}
+      ${GIT_EXECUTABLE} ${git_repository} ${git_tag} ${git_remote_name} "${git_submodules}" ${src_name} ${work_dir}
       ${stamp_dir}/${name}-gitinfo.txt ${stamp_dir}/${name}-gitclone-lastrun.txt
       )
     set(comment "Performing download step (git clone) for '${name}'")
@@ -1993,9 +2000,13 @@ function(_ep_add_update_command name)
     if(NOT git_tag)
       set(git_tag "master")
     endif()
+    get_property(git_remote_name TARGET ${name} PROPERTY _EP_GIT_REMOTE_NAME)
+    if(NOT git_remote_name)
+      set(git_remote_name "origin")
+    endif()
     get_property(git_submodules TARGET ${name} PROPERTY _EP_GIT_SUBMODULES)
     _ep_write_gitupdate_script(${tmp_dir}/${name}-gitupdate.cmake
-      ${GIT_EXECUTABLE} ${git_tag} "${git_submodules}" ${git_repository} ${work_dir}
+      ${GIT_EXECUTABLE} ${git_tag} ${git_remote_name} "${git_submodules}" ${git_repository} ${work_dir}
       )
     set(cmd ${CMAKE_COMMAND} -P ${tmp_dir}/${name}-gitupdate.cmake)
     set(always 1)