gnm: avoid redundant root path cost re-summation in KShortestPaths

[KinshukSS2]

description

the current method to find costs was that whenever a path was found then it usually checks the cost from the start thus making it less efficient

the fix i propose usually only plays a better role when there is a presence of long paths in large GNM networks
it basically only finds the cost for the new branches whenever created

fixes #14090

[rouault]

CC @BishopGIS

[KinshukSS2]

any updates on the status of the pr...pls let me know if any changes are required from my side

[jratike80]

any updates on the status of the pr...

Waiting for someone to review. You know, this is open source, there are not many people who know gnm well, and there are also other things going on https://github.com/OSGeo/gdal/pulse.

[rouault]

@BishopGIS Still available for GNM related topics ?

[github-actions]

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