luci.mk: add reproducibility by setting specific version hash length

luci.mk: add reproducibility by setting specific version hash length

The `git branch` feature may return different hash length, resulting in
reproducibility issues.

│ │ ├── ./usr/share/ucode/luci/version.uc
│ │ │ @@ -1 +1 @@
│ │ │ -export const revision = '26.133.20346~e9ebca7', branch = 'LuCI (HEAD detached at e9ebca7) branch';
│ │ │ +export const revision = '26.133.20346~e9ebca7', branch = 'LuCI (HEAD detached at e9ebca75) branch';

Since `git branch` does not support `--abbrev=7` like `git log` does, use `-c
core.abbrev=7`.

Signed-off-by: Paul Spooren <redacted>