From: h8d13 <hadean-eon-dev@proton.me>

When git clone is run by a tool the user does not control directly
(CI runners, package build scripts such as makepkg, or any wrapper
that spawns nested clones), there is no way to request a shallow
clone: --depth only exists as a command-line option on the process
that invokes git clone, and unlike url.*.insteadOf there is no
configuration key that could be injected via GIT_CONFIG_* to achieve
the same effect.

Teach git clone to read a DEPTH environment variable when --depth is
not given on the command line. Since environment variables propagate
to child processes, exporting DEPTH=1 once makes every nested clone
underneath shallow, which is useful in CI pipelines and recursive
build tools. An explicit --depth on the command line still takes
precedence, and the value goes through the existing validation, so a
non-positive DEPTH dies with the same error as a non-positive
--depth.

Signed-off-by: h8d13 <hadean-eon-dev@proton.me>

Submitted-As: https://lore.kernel.org/git/pull.2333.git.git.1781314780645.gitgitgadget@gmail.com