Limit the branch name length of git trac

[koffie]

Currently git trac creates extremely long branch names like at for example: making_disjointunionenumeratedsets_check_for_finiteenumeratedsets_when_category_is_not_set at #21218 . This not only renders really awkwardly on trac, but is also highly inconvenient for people using git the hard way.

[vbraun]

You probably want bash tab-completion for git branches; The actual length is irrelevant as you ever only type the first couple of chars.

Having said that, I don't mind merging a patch that limits the default length (but something reasonably long, say, 30 chars?)