This is one of the challenges of developing SharePoint. We do not store the “content” in the source control, everything that is included in this code includes everything that we add to _layouts. Content (everything in the content database) is still an integral part of the “solution”. We use the version built into SharePoint to do this, and the integration server contains the original copy for everyone to get copies. Content is not deployed in the same way as code, so it is not included in the assembly.
In some projects, this may be possible using export and import or backup; each developer checks the export of a site that uses the solution, and it can be restored in other development environments. This will not allow merging and rewriting the developers' own content.
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