tigefa4u projects Trac

When Trac uses SQLite3/PySQLite2 for its DatabaseBackend, concurrent writes
were not handled well: this could lead to persistent database locks
and, occasionally, to crashes (see #2170).

With the present changes, the issue is fixed, ''provided'' the SQLite library
which is used was compiled in thread-safe mode.
This is always the case on Windows, but Linux users (and probably all Unix users)
will certainly have to recompile their SQLite package (3.2.7 preferred)
using the `--enable-threadsafe` configuration option.

Note that ''without'' a threadsafe SQLite, Trac ''will'' continue to work,
simply it will still be vulnerable to the abovementioned problem.

Finally, note that ''without'' this patch, Trac simply won't work with
a threadsafe SQLite binary on Linux (see #2196).

The browser (and the log revision) module now return a proper error message
when the requested path doesn't exist at the specified revision.

Additionally, from that error page, the user can follow a link to the
revision log in ''path history'' mode that will show him the delete operation
which eventually removed the requested path from the repository,
or another error message stating that the requested path doesn't exist in any older
revision in the repository.

Don't prefix an absolute path for `log_file` with the environment `log` directory.

WebAdmin: add page for logging configuration

`os.path.samefile()` is not implemented on Windows, so use a different approach in `trac.loader`.

Fix unit tests broken in [2349].

Automatically enable plugins loaded from the environment plugins directory. Can of course still be disabled explicitly.

WebAdmin: Fix enabling/disabling components. Closes #2204.

Lowercase classname for component enablement check, to fix enabling built-in components in WebAdmin.

Display download link instead of empty content table for binary attachments/files. Fixes #2144.