On Windows, try to update to a branch which contains characters
outside the current mbcs, e.g. "fossil update eclipse-project".
Below is a log what happens.

Branch "unicode" [8596323b5b] contains my minimal proposed
fix. It is tested on UNIX and Windows.

Here is what happens (Windows only!)
>fossil update eclipse-project
.....
ADD .project (overwrites an unmanaged file)
ADD .settings/org.eclipse.core.runtime.prefs
UPDATE src/export.c
UPDATE src/file.c
UPDATE src/finfo.c
UPDATE src/import.c
UPDATE src/json.c
UPDATE src/main.c
UPDATE src/merge.c
UPDATE src/name.c
UPDATE src/popen.c
UPDATE src/rebuild.c
UPDATE src/shell.c
MERGE src/timeline.c
UPDATE src/translate.c
UPDATE src/vfile.c
UPDATE src/winhttp.c
ADD test/??/??.txt
C:\Localdata\workspace\fossil\fossil.exe: unable to create directory C:/Localdat
a/workspace/fossil/test/??
Rolling back prior filesystem changes...
UNDO .project
DELETE .settings/org.eclipse.core.runtime.prefs
UNDO src/export.c
UNDO src/file.c
UNDO src/finfo.c
UNDO src/import.c
UNDO src/json.c
UNDO src/main.c
UNDO src/merge.c
UNDO src/name.c
UNDO src/popen.c
UNDO src/rebuild.c
UNDO src/shell.c
UNDO src/timeline.c
UNDO src/translate.c
UNDO src/vfile.c
UNDO src/winhttp.c
DELETE test/??/??.txt
>

Cannot update when branch contains non-mbcs filenames