Split git setup to several steps

Configuration requirements for `git` and `git_am` can be different
enough so that that these two should not share the setup routine
in it's entirety [1]. On the other hand, some parts of the setup
can (and should) be kept same in both approaches.

[1]: #3472

This splits the `%__scm_setup_git` macro into several named steps,
that are later combined. This will allow for future differentiation
of `git` and `git_am` by customizing (only) the relevant step(s).

macros.in

@@ -1239,17 +1239,26 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
 %__scm_source_timestamp @${SOURCE_DATE_EPOCH:-${RPM_BUILD_TIME:?}}
 
 # Git
-%__scm_setup_git(q)\
+%__scm_init_git(q)\
 %{__git} init %{-q}\
+%{__git} config gc.auto 0
+
+%__scm_configure_git\
 %{__git} config user.name "%{__scm_username}"\
-%{__git} config user.email "%{__scm_usermail}"\
-%{__git} config gc.auto 0\
+%{__git} config user.email "%{__scm_usermail}"
+
+%__scm_import_git(q)\
 %{__git} add --force .\
 GIT_COMMITTER_DATE=%{__scm_source_timestamp} GIT_AUTHOR_DATE=%{__scm_source_timestamp}\\\
 	%{__git} commit %{-q} --allow-empty -a\\\
 	--author "%{__scm_author}" -m "%{NAME}-%{VERSION} base"\
 %{__git} checkout --track -b rpm-build
 
+%__scm_setup_git(q)\
+%{expand:%__scm_init_git %{-q}}\
+%{expand:%__scm_configure_git}\
+%{expand:%__scm_import_git %{-q}}
+
 %__scm_apply_git(qp:m:)\
 %{__git} apply --index --reject %{-p:-p%{-p*}} -\
 GIT_COMMITTER_DATE=%{__scm_source_timestamp} GIT_AUTHOR_DATE=%{__scm_source_timestamp}\\\