git: Skip checks whether a fetch is updating submodules

When fetching into a repository, Git will by default check whether the
fetch brings in any commits which update a submodule. If so, and if that
submodule is populated, then we'll recurse into that submodule and also
perform a fetch in there. This is useless though in our context because
we never populate submodules, so computing whether there are commits
which update any submodule is a complete waste of time.

The following mirror-fetch has been executed in www-gitlab-com:

    Benchmark 1: git fetch --recurse-submodules=on-demand +refs/*:refs/*
      Time (mean ± σ):     66.595 s ±  1.396 s    [User: 63.019 s, System: 8.729 s]
      Range (min … max):   65.377 s … 68.118 s    3 runs

    Benchmark 2: git fetch --recurse-submodules=no +refs/*:refs/*
      Time (mean ± σ):     62.789 s ±  1.202 s    [User: 61.434 s, System: 7.774 s]
      Range (min … max):   61.621 s … 64.022 s    3 runs

    Summary
      'git fetch --recurse-submodules=no +refs/*:refs/*' ran
        1.06 ± 0.03 times faster than 'git fetch --recurse-submodules=on-demand +refs/*:refs/*'

This demonstrates that we get a nice small speedup by explicitly
disabling the check.

1 files changed, 9 insertions, 0 deletions

diff --git a/internal/git/command_description.go b/internal/git/command_description.go
index 3efad9c5c..cb26d08d5 100644
--- a/internal/git/command_description.go
+++ b/internal/git/command_description.go
@@ -98,6 +98,15 @@ var commandDescriptions = map[string]commandDescription{
 			// us and unreachable from the outside, this is dangerous. We thus have to
 			// disable redirects in all cases.
 			ConfigPair{Key: "http.followRedirects", Value: "false"},
+
+			// By default, Git will try to recurse into submodules on demand: if a fetch
+			// retrieves a commit that updates a populated submodule, then it recurses
+			// into that submodule and also updates it. Computing this condition takes
+			// some resources though given that we need to check all fetched commits to
+			// find out if any submodule was in fact updated. This is a complete waste
+			// of time though because we never populate submodules at all. We thus
+			// disable recursion into submodules.
+			ConfigPair{Key: "fetch.recurseSubmodules", Value: "no"},
 		}, fsckConfiguration("fetch")...),
 	},
 	"for-each-ref": {