g.extension: use git sparse-checkout instead of SVN (OSGeo#2895)

* unified imports
* use git sparse checkout
* version dependent fall-back
* test on mac

.github/workflows/macos_gunittest.cfg

@@ -24,7 +24,6 @@ exclude =
     raster/r.in.pdal/testsuite/test_r_in_pdal_selection.py
     raster/r.terraflow/testsuite/test_r_terraflow.py
     raster/r.sun/testsuite/test_rsun.py
-    scripts/g.extension/testsuite/test_addons_download.py
     scripts/g.search.modules/testsuite/test_g_search_modules.py
     temporal/t.connect/testsuite/test_distr_tgis_db_raster3d.py
     temporal/t.connect/testsuite/test_distr_tgis_db_raster.py

scripts/g.extension/g.extension.html

@@ -75,21 +75,23 @@ <h4>Local source code directory</h4>
 To keep the directory clean, the directory content is copied
 to a temporary directory and the compilation happens there.
 
-<h4>Local source code ZIP file</h4>
+<h4>Source code in a ZIP or TAR archive</h4>
 In addition, new extension can be also installed from a ZIP file
 or an archive file from the TAR family (e.g., <tt>.tar.gz</tt> or <tt>.bz2</tt>).
-The file can be on disk (specified with a path), or on web (specified by
+The file can be on disk (specified with a path), or on the web (specified by
 an URL).
 
 <h4>Online repositories: GitHub, GitLab and Bitbucket</h4>
 For well known general hosting services, namely GitHub, GitLab and Bitbucket,
-<em>g.extension</em> supports the download of a repository as a ZIP file.
+<em>g.extension</em> supports the download of a repository.
 Here the user only needs to provide a base URL to the repository web page
 (with or without the <tt>https://</tt> part).
-For GitLab and Bitbucket, the latest source code in the default branch is
-downloaded, for GitHub, the latest source code in the master branch is downloaded.
-Of course, a user can still specify the full URL of a ZIP file
-and install a specific branch or release in this way (ZIP file mechanism
+For GitHub, GitLab and Bitbucket, the latest source code in the default
+branch is downloaded, unless a specific branch is requested in the
+<em>branch</em> option.
+
+Of course, a user can still specify the full URL of a ZIP file e.g. for a
+specific release and install the achived code in this way (ZIP file mechanism
 will be applied).
 
 <p>
@@ -98,12 +100,12 @@ <h4>Online repositories: GitHub, GitLab and Bitbucket</h4>
 depend on a specific infrastructure.
 For other sources and repositories, this is not supported because it is
 assumed that other sources contain only one extension, typically a module
-or group of modules.
+or group of modules with a Makefile at the root of the repository.
 
 <h4>Needed directory layout</h4>
 When none of the above sources is identified, <em>g.extension</em> assumes
 <!-- TODO: update this description -->
-that the source is in a GitHub repository and uses the <em>svn</em> command
+that the source is in a GitHub repository and uses the <em>git</em> command
 line tool to obtain the source code. The expected structure of the repository
 should be the same as the one of the official repository.
 
@@ -225,6 +227,18 @@ <h3>Installing when writing a module locally</h3>
 g.extension r.example url=/local/directory/r.example/
 </pre></div>
 
+<h2>REQUIREMENTS</h2>
+In order to clone source code repositories, the <em>git</em> command line
+tool is required. The installation of single AddOns is most efficient with
+versions of git that support so called <i>sparse checkout</i>, which was
+introduced with version 2.25. With older versions of git, the entire AddOns
+repository will be downloaded.
+
+On UNIX like systems, installation is done with the
+<em>make</em> command line tool. For AddOns written in C / C++, a respective
+build environment is needed.
+
+
 <h2>KNOWN ISSUES</h2>
 
 Toolboxes in the official repository cannot be downloaded.