tigefa4u projects Trac

In TracTimeline, show the common location for the file entries of changeset events.

This is a follow-up of what r4676 did for the TracChangeset view itself.

The existing TracIni setting `changeset_show_files` is reused:
{{{
[timeline]
changeset_show_files = location
}}}

Implements #3274.

In TracChangeset, show the common location for the file entries.

This will be quite useful for those having deep paths in their repositories, as usually changesets happen below some specific folder in the repository only.
That folder will now be shown as the '''Location:''', and the '''Files:''' section will only contain the paths relative to that Location.

Implements #3549.

Fix a regression introduced by r4668: property changes were not shown in the absence of text diffs.

Tabular/Unified js switch: `substr(5)` was truncating the file names. Fixes #4326.

0.10.4dev: ported r4672 (''improve vcs not supported error message, #4635'')

Improve error message for the much too common ''Unsupported version control system'' error. Fixes #4635.

Cosmetic clean-ups in trac.env for !TracError

0.10.4dev: follow-up to r4664, merged r4669.

Follow-up to r4662: we need to let the ''The 'repository_dir' has changed'' error go through.

Some cosmetic improvements for the TracChangeset view:
- show legend for file changes before the list of files (#813)
- add the file count to the file legend, e.g. "- 5 added - 3 modified" (#3104)