Store empty (default) branch in branch cache, too.
Operations on this branch don't work otherwise.
Reading branches.cache had to be adjusted to allow an empty label.
Adjusted reading of the cache tip for symmetry, no functional change here.
adding foo
adding bar
adding baz/bletch
test-archive-TIP/.hg_archival.txt
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
test-archive-TIP/.hg_archival.txt
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
Archive: archive.zip
testing: test-archive-TIP/.hg_archival.txt OK
testing: test-archive-TIP/bar OK
testing: test-archive-TIP/baz/bletch OK
testing: test-archive-TIP/foo OK
No errors detected in compressed data of archive.zip.
test/.hg_archival.txt
test/bar
test/baz/bletch
test/foo
test/.hg_archival.txt
test/bar
test/foo
test-TIP/.hg_archival.txt
test-TIP/bar
test-TIP/baz/bletch
test-TIP/foo
abort: archive prefix contains illegal components
Archive: test.zip
testing: test/.hg_archival.txt OK
testing: test/bar OK
testing: test/baz/bletch OK
testing: test/foo OK
No errors detected in compressed data of test.zip.
test-TIP/.hg_archival.txt
test-TIP/bar
test-TIP/baz/bletch
test-TIP/foo