[feature] self checkout from a branch

[andry81]

Sometimes need to checkout a branch that is already downloaded into the repository because that branch is a child directory to another repository branch:

repositories:

  _externals/3_14_x:
    type:         git
    url:          .
    version:      3_14_x

  _externals/3_15_x:
    type:         git
    url:          .
    version:      3_15_x

, where the 3_x is a parent (master) branch containing that .externals file.

Because 3_14_x and 3_15_x already downloaded, then I don't want to download them again and want to use current repository in the url: .

Or, for example, I could fetch only 3_14_x and don't want to fetch 3_15_x because each branch could weight hundreds of megabytes and I don't want to fetch them all in vcs import ....

Or may be more advanced syntax:

repositories:

  _externals/3_14_x:
    type:             git
    url:              <path-to-remote-repo-1>
    version:          3_14_x
    fetch-to-branch:  3_14_x

  _externals/3_15_x:
    type:             git
    url:              <path-to-remote-repo-1>
    version:          3_15_x
    fetch-to-branch:  3_15_x

, where fetch-to-branch will override default fetch and will checkout from the local branch.

Additionally add command line options with wildcards:

To exclude:

>vcs import --skip-fetch-branches "3_14_*|3_15_*" < .externals

To exclusive include:

>vcs import --checkout-branches "3_15_*|3_16_*" < .externals

Mixed (to rely on external fetch):

>vcs import --skip-fetch-branches '*' --checkout-branches "3_15_*|3_16_*" < .externals

Is there a chance to add this?