Use explicit default branch in %autosetup -S git

Commit 3a6b1d8fbf assumed "master" as the
default git branch to track, however the name can be changed globally,
so make it explicit.

Given the choice now, go with "main" as the general trend, e.g:
https://fedoraproject.org/wiki/Changes/GitRepos-master-to-main

macros.in

@@ -1163,7 +1163,7 @@ package or when debugging this package.\
 
 # Git
 %__scm_setup_git(q)\
-%{__git} init %{-q}\
+%{__git} init %{-q} --initial-branch=main\
 %{__git} config user.name "%{__scm_username}"\
 %{__git} config user.email "%{__scm_usermail}"\
 %{__git} config gc.auto 0\
@@ -1172,7 +1172,7 @@ package or when debugging this package.\
 	--author "%{__scm_author}" -m "%{NAME}-%{VERSION} base"\
 %{__git} branch rpm-build \
 %{__git} checkout rpm-build \
-%{__git} branch --set-upstream-to=master
+%{__git} branch --set-upstream-to=main
 
 %__scm_apply_git(qp:m:)\
 %{__git} apply --index --reject %{-p:-p%{-p*}} -\