[RFC PATCH v3 1/2] git-submodule.sh:cmd_update: if submodule branch exists, fetch that instead of default

["Eddy Petrișor" <eddy.petrisor@codeaurora.org>]

From: Eddy Petrișor <eddy.petrisor@gmail.com>

There are projects such as llvm/clang which use several repositories, and they
might be forked for providing support for various features such as adding Redox
awareness to the toolchain. This typically means the superproject will use
another branch than master, occasionally even use an old commit from that
non-master branch.

Combined with the fact that when incorporating such a hierachy of repositories
usually the user is interested in just the exact commit specified in the
submodule info, it follows that a desireable usecase is to be also able to
provide '--depth 1' or at least have a shallow clone to avoid waiting for ages
for the clone operation to finish.

In theory, this should be straightforward since the git protocol allows
fetching an arbitary commit, but, in practice, some servers do not permit
fetch-by-sha1.

Git submodule seems to be very stubborn and cloning master, although the
wrapper script and the gitmodules-helper could work together to clone directly
the branch specified in the .gitmodules file, if specified.

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>
---
 git-submodule.sh | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/git-submodule.sh b/git-submodule.sh
index 24914963c..65e3af08b 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -589,8 +589,10 @@ cmd_update()
 			branch=$(git submodule--helper remote-branch "$sm_path")
 			if test -z "$nofetch"
 			then
+				# non-default branch refspec
+				br_refspec=$(git submodule-helper remote-branch $sm_path)
 				# Fetch remote before determining tracking $sha1
-				fetch_in_submodule "$sm_path" $depth ||
+				fetch_in_submodule "$sm_path" $depth $br_refspec ||
 				die "$(eval_gettext "Unable to fetch in submodule path '\$sm_path'")"
 			fi
 			remote_name=$(sanitize_submodule_env; cd "$sm_path" && get_default_remote)
-- 
2.16.2