bug#40377: guix build --with-commit is broken

[Brice Waegeneire <brice@waegenei.re>]

Hello,

`--with-commit' from `guix build' doesn't always work. It seems to not
really update the checkout contrary to what is reported since changing 
the
`git-url`, by adding a slash for example, put it back on track. It makes 
it
useless when trying out to build a newly released version. Following are
some examples:

$ guix build mlt
/gnu/store/m6y21nnyg6p0kcxbfj7k1dnbwbsbj006-mlt-6.18.0
$ guix build mlt --with-commit=mlt=v6.18.0
updating checkout of 'https://github.com/mltframework/mlt.git'...
guix build: error: Git failure while fetching 
https://github.com/mltframework/mlt.git: the requested type does not 
match the type in the ODB
$ guix build mlt --with-commit=mlt=v6.18.0 
--with-git-url=mlt=https://github.com/mltframework/mlt.git/
updating checkout of 'https://github.com/mltframework/mlt.git/'...
retrieved commit 46a226217d8ec5bbf66a4fa3da138fd6924dd13b
The following derivations will be built:
    /gnu/store/ljlj1dwkdfb7sziy02hi5scskdjmxv02-mlt-6.18.0.drv
    /gnu/store/02q25aq6mi8287hbakc9v7z0jmq9f6cz-sdl-1.2.15.drv
    /gnu/store/5z3v3w5aslbcsq0ia429n0pkshv6f14x-frei0r-plugins-1.7.
[...]

$ guix build picom --with-commit=picom=v7.5
updating checkout of 'https://github.com/yshui/picom.git'...
retrieved commit 3838f053afb1aa05ba1ab489f2dfaa44c00c9ef0
/gnu/store/q1bl4q1x4wmyps5ibljvj4x3y2nl24la-picom-7.5
$ guix build picom --with-commit=picom=vNext
updating checkout of 'https://github.com/yshui/picom.git'...
guix build: error: Git failure while fetching 
https://github.com/yshui/picom.git: the requested type does not match 
the type in the ODB

I've found out about this issue by reading pelzflorian's suggestion 
about
adding a `--with-version' in [0] and [1], he wrote that a year ago, 
before
support for tags was added to `--with-commit'.

[0]: https://issues.guix.info/issue/35744
[1]: https://lists.gnu.org/archive/html/help-guix/2019-05/msg00093.html

- Brice