Implement REPREPRO_GIT_BRANCH for reprepro importer

[j-rivero]

Expose REPREPRO_GIT_BRANCH variable as a Jenkins parameter

[j-rivero]

Tested in ros-infrastructure/reprepro-updater#193

Diff:

diff -ur -I '.*<id>dashboard_portlet_.*</id>.*' /tmp/current_xml_configuration/reprepro_importer.xml /tmp/pr_xml_configuration/reprepro_importer.xml
--- /tmp/current_xml_configuration/reprepro_importer.xml	2024-01-09 17:45:46.792857439 +0000
+++ /tmp/pr_xml_configuration/reprepro_importer.xml	2024-01-09 17:44:24.084328232 +0000
@@ -21,6 +21,11 @@
                     <description>Repository to host the new imported packages. Should match entry in reprepro.ini</description>
                 </hudson.model.StringParameterDefinition>
                 <hudson.model.StringParameterDefinition>
+                    <name>REPREPRO_GIT_BRANCH</name>
+                    <defaultValue></defaultValue>
+                    <description>Branch of the repository to be used for code and REPREPRO_IMPORT_YAML_FILE (empty implies uses default)</description>
+                </hudson.model.StringParameterDefinition>
+                <hudson.model.StringParameterDefinition>
                     <name>REPREPRO_IMPORT_YAML_FILE</name>
                     <defaultValue></defaultValue>
                     <description>Name of the reprepo import file</description>