guix build: Add '--with-commit'.

* guix/git.scm (<git-checkout>)[commit]: New field.
(git-checkout-compiler): Honor it.
* guix/scripts/build.scm (evaluate-git-replacement-specs): Add 'proc'
parameter and honor it.
(transform-package-source-branch)[replace]: New procedure.
Adjust 'evaluate-git-replacement-specs' accordingly.
(transform-package-source-commit): New procedure.
(%transformations, %transformation-options)
(show-transformation-options-help): Add 'with-commit'.
* tests/guix-build-branch.sh: Add test.
* doc/guix.texi (Package Transformation Options): Document it.