revset: for x^2, do not take null as a valid p2 revision Since we don't count null p2 revision as a parent, x^2 should never return null even if null is explicitly populated.

revset: make follow() reject more than one start revisions Taking only the last revision is inconsistent because ancestors(set) follows all revisions given, and theoretically follow(startrev=set) == ancestors(set). I'm planning to add a support for multiple start revisions, but that won't fit to the 4.0 time frame. So reject multiple revisions now to avoid future BC. len(revs) might be slow if revs were large, but we don't care since a valid revs should have only one element.

bundle2: only emit compressed chunks if they have data This is similar to 58467204cac0. Not all calls into the compressor return compressed data, as the compressor may buffer compressed output internally. It is cheaper to check for empty chunks than to send empty chunks through the generator. When generating a gzip-v2 bundle of the mozilla-unified repo, this change results in 50,093 empty chunks not being sent through the generator (out of 1,902,996 total input chunks).

color: add some documentation for custom terminfo codes

color: debugcolor should emit the user-defined colors This also fixes a long-standing bug that reversed the sense of the color name and the label used to print it, which was never relevant before.

color: ignore effects missing from terminfo If terminfo mode is in effect, and an effect is used which is missing from the terminfo database, simply silently ignore the request, leaving the output unaffected rather than causing a crash.