This fixes a minor inaccuracy issue when creating git checkout commands
for branches.
The git checkout command used within `GitDownloader::updateToCommit()`
does not use the "`--`" sequence to separate branch from file parameters.
This leads to an inaccuary as git tries as well the branch name as file
name. If the non-existent branch is actually the name of a file, the file
is checked out. If the non-existent branch is not the name of a file, an
error message is given:
> error: pathspec 'non-existent-branch' did not match any file(s) known to
git.
Both cases are not expected for the program flow in
`GitDownloader::updateToCommit()`.
The only thing that is expected is a non-existent branch to fail to
checkout - but with a different error message:
> // reference was not found (prints "fatal: reference is not a tree:
$ref")
This can be easily fixed by adding the missing separator when constructing
the command which is applied with this commit.
7077803aa4