submodule helper: convert relative URL to absolute URL if needed

submodule helper: convert relative URL to absolute URL if needed

The submodule helper update_clone called by "git submodule update",
clones submodules if needed. As submodules used to have the URL indicating
if they were active, the step to resolve relative URLs was done in the
"submodule init" step. Nowadays submodules can be configured active without
calling an explicit init, e.g. via configuring submodule.active.

When trying to obtain submodules that are set active this way, we'll
fallback to the URL found in the .gitmodules, which may be relative to the
superproject, but we do not resolve it, yet:

    git clone https://gerrit.googlesource.com/gerrit
    cd gerrit && grep url .gitmodules
	url = ../plugins/codemirror-editor
	...
    git config submodule.active .
    git submodule update
    fatal: repository '../plugins/codemirror-editor' does not exist
    fatal: clone of '../plugins/codemirror-editor' into submodule path '/tmp/gerrit/plugins/codemirror-editor' failed
    Failed to clone 'plugins/codemirror-editor'. Retry scheduled
    [...]
    fatal: clone of '../plugins/codemirror-editor' into submodule path '/tmp/gerrit/plugins/codemirror-editor' failed
    Failed to clone 'plugins/codemirror-editor' a second time, aborting
    [...]

To resolve the issue, factor out the function that resolves the relative
URLs in "git submodule init" (in the submodule helper in the init_submodule
function) and call it at the appropriate place in the update_clone helper.

Reported-by: Jaewoong Jung <jungjw@google.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>