git-recursive: resolve relative submodule URLs

[marvil6]

git-recursive.sh passes each submodule's .gitmodules URL straight to git clone --mirror / git remote set-url. When that URL is relative (e.g. url = ../system.git), git treats it as a filesystem path instead of resolving it against the parent repository's own URL, so the clone fails. The return value of the recursive git_sync_recursive call is never checked, so the job still reports success while those submodules are left empty. This is the root cause identified in #161 by @ZenithalHourlyRate.

What changed

• Add resolve_submodule_url(). A URL starting with ./ or ../ is resolved against the parent repository's URL the way git does (see gitsubmodules(7)). Every other form (https://, ssh://, scp-like, file://, absolute paths, ...) passes through unchanged.
• checkout_repo takes the current repository's own upstream URL as a fourth argument (the caller already has it as $upstream) and resolves each submodule URL before recursing. Nested submodules resolve against their already-resolved parent.
• If a relative URL climbs above what the parent URL can supply, the submodule is skipped with a message naming both URLs. In that case git's own resolver keeps eating into the scheme/authority and returns a malformed URL, so this patch refuses instead of reproducing it (see the table below).

Results

offline, file:// repos. Columns are exit status / gitlinks in the parent / submodule mirrors populated:

Fixture	Before	After
Flat, 5 relative submodules	0 / 5 / 0	0 / 5 / 5
Flat, 50 relative submodules	0 / 50 / 0	0 / 50 / 50
Nested (relative submodule with its own relative submodule)	0 / 1 / 0 of 2	0 / 1 / 2 of 2
./-relative submodule	0 / 1 / 0	0 / 1 / 1
Mixed relative + absolute	0 / 2 / 1	0 / 2 / 2
One relative + one genuinely unreachable	0 / 2 / 0	0 / 2 / 1 (the unreachable one still fails)
Absolute-URL control	0 / 5 / 5	0 / 5 / 5

For the absolute-URL control, the generated client script is identical before and after. A fresh git clone --recurse-submodules from the patched output has every submodule at its pinned commit.

Real world: boostorg/boost (all 173 submodules use relative URLs):

	Before	After
Exit status	0	0
Submodule mirrors populated	0 of 173	173 of 173
Wall time	20 s (superproject only)	458 s (173 additional clones)
Mirror size	217 MB	2.2 GB

Environment and caveats

• Tested on a Linux host with bash 5.2.21, git 2.43.0 and GNU sed 4.9, and re-run inside the tunathu/tunasync-scripts image itself (bash 5.2.37, git 2.47.3) with identical results.
• shellcheck reports no new warnings on the changed lines.
• The resolver is validated against git's own resolution and against boostorg/boost, not every URL scheme git accepts.

Fixes #161