Fixed incorrect relative submodule URLs

[goriy]

According to git submodule documentation base path for relative submodule URL
is parent repository's root - not submodule "mount point".

So it's necessary to know "path" of parent repository to create correct submodule
URL.

Unfortunately, RefUrl function semantics are changed (kind of API change), so it can break something else that uses it.
At least it is required to change /templates/repo/view_list.tmpl in gogs project.

For more details see issue gogs/gogs#1926

Thus, maybe it's even better to make new function (temporary or permanently) similar to RefUrl (I don't know how bad it is to break compatibility in this particular case).

[unknwon]

Thanks your PR!

Any input/output examples?

[goriy]

As an example: https://try.gogs.io/illustr/repo1/src/master/shared

Submodule repo2 has the relative URL: ../repo2

Let's assume that file https://github.com/gogits/gogs/blob/master/templates/repo/view_list.tmpl is patched according to gogs/gogs#1926 (i. e. RefUrl on lines 33,34 is called in such way: $commit.RefUrl AppUrl $.BranchLink)

While composing submodule URL, function RefUrl is called with arguments:

• urlPrefix = "https://try.gogs.io/"
• parentPath = "/illustr/repo1/src/master/shared"

Variable sf.refUrl inside function contains "../repo2".

So output will be constructed like this: "https://try.gogs.io" + "/illustr/repo1" + "/" + "../repo2"

Thus, returned value is "https://try.gogs.io/illustr/repo1/../repo2". Corresponding absolute url is "https://try.gogs.io/illustr/repo2" as expected.

I've never used Go language before, so I'm not sure that it's the best solution.