clone: accept DEPTH env var as fallback for --depth

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>

Author: h8d13

builtin/clone.c

@@ -1022,6 +1022,12 @@ int cmd_clone(int argc,
 		usage_msg_opt(_("You must specify a repository to clone."),
 			builtin_clone_usage, builtin_clone_options);
 
+	if (!option_depth) {
+		const char *env_depth = getenv("DEPTH");
+		if (env_depth && *env_depth)
+			option_depth = xstrdup(env_depth);
+	}
+
 	if (option_depth || option_since || option_not.nr)
 		deepen = 1;
 	if (option_single_branch == -1)