Fix head reference in zig fetch commands

[AloisMahdal]

This syntax:

URL?ref=REF

is what ends up in the build.zig.zon, but does not work when used on the command line. What works on command line is:

URL#REF

E.g. following test:

$ grep 'rock\|zeit-' build.zig.zon
$ zig fetch --save git+https://github.com/rockorager/zeit?ref=main
info: resolved to commit 905396257dcf0b1bfa3c0e8265657a0e748105ad
$ grep 'rock\|zeit-' build.zig.zon
            .url = "git+https://github.com/rockorager/zeit?ref=main#905396257dcf0b1bfa3c0e8265657a0e748105ad",
            .hash = "zeit-0.10.0-dev-5I6bk3m9AgB8Ndev81pibV70MJe9VCOXAGZBNU833c4Y",
$ zig fetch --save git+https://github.com/rockorager/zeit?ref=v0.9.0
info: resolved to commit 905396257dcf0b1bfa3c0e8265657a0e748105ad
warning: overwriting existing dependency named 'zeit'
$ grep 'rock\|zeit-' build.zig.zon
            .url = "git+https://github.com/rockorager/zeit?ref=v0.9.0#905396257dcf0b1bfa3c0e8265657a0e748105ad",
            .hash = "zeit-0.10.0-dev-5I6bk3m9AgB8Ndev81pibV70MJe9VCOXAGZBNU833c4Y",
$ zig fetch --save git+https://github.com/rockorager/zeit#v0.9.0
info: resolved ref 'v0.9.0' to commit b1c1c2fcbc71fd7799a316bbcf0ff88d06d80ccc
warning: overwriting existing dependency named 'zeit'
$ grep 'rock\|zeit-' build.zig.zon
            .url = "git+https://github.com/rockorager/zeit?ref=v0.9.0#b1c1c2fcbc71fd7799a316bbcf0ff88d06d80ccc",
            .hash = "zeit-0.9.0-5I6bk2m9AgBSMH8-L6rYJkwuQAyhXplnfxnvTSGzVHUR",
$

shows that:

1. on empty build.zig.zon, using ?ref=main fetches main

2. then ?ref=v0.9.0 still fetches main, it just updates URL but leaves commit reference and package hash untouched

3. correct form #v0.9.0 will update URL, commit reference and package hash.

[AloisMahdal]

Note that I've reported the zig fetch "footgun" to zig: https://codeberg.org/ziglang/zig/issues/35756