libfetchers/git: add support for '.' in gitmodules

[fzakaria]

Motivation

Please see #13215

Basically, .gitmodules file may have . as the branch listed for a submodule.
The git-documentation claims this is a "special" branch that represents it should be identical to the parent repository. [ref]

Validation

broken

[----------] 1 test from GitTest
[ RUN      ] GitTest.submodule_period_support
unknown file: Failure
C++ exception with description "error:
       … while fetching the input 'git+file:///tmp/nix-120109-2774730724/repo?ref=main&submodules=1'

       error: invalid Git branch/tag name '.'" thrown in the test body.

[  FAILED  ] GitTest.submodule_period_support (20 ms)
[----------] 1 test from GitTest (20 ms total)

fixed

meson test -C build nix-fetchers-tests
ninja: Entering directory `/home/fmzakari/code/github.com/NixOS/nix/build'
[2/2] Linking target src/libfetchers-tests/nix-fetchers-tests
1/1 libfetchers-tests / nix-fetchers-tests        OK              0.08s

Alternatively, I made a demo repo with this feature.

> cat /tmp/test/.gitmodules
[submodule "nix-auto-follow"]
	path = nix-auto-follow
  url = https://github.com/fzakaria/nix-auto-follow.git
  branch = .

> _NIX_FORCE_HTTP=1 ./build/src/nix/nix eval --impure --expr 'builtins.fetchGit { url="file:///tmp/test"; submodules=true; }'
{ lastModified = 1755229090; lastModifiedDate = "20250815033810"; narHash = "sha256-yNOsLiJAtS/atdWMto3usCEBmerWonCVoe557zQRvt4="; outPath = "/nix/store/3mcd25dyi8nfz84zxlz8v3bg0ch9crbz-source"; rev = "9e37ffc3ab465887b20be4ac42224e483706cefc"; revCount = 1; shortRev = "9e37ffc"; submodules = true; }

fixes #13215

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[xokdvium]

I think @Ericson2314 had a old PR to make dummy:// use more in-memory stuff. Would that be useful here? That never got merged.

[xokdvium]

This one #10915

[fzakaria]

Discussed on matrix.
I will like to pick it up; seems worthwhile -- I might also make it a separate store.

Either way I think it doesn't block this PR.