refactor(fossil): use `fossil branch current` supported since v2.7 (#11138)
refactor(fossil): use `fossil branch current` supported since v2.7 (#11138)

Using the automation-friendly "fossil branch current" feature added in
Fossil 2.7 instead of ad hoc parsing of human-readable "fossil branch"
output.  Not only does this fix a stray space in the output, it's more
robust against changes in command output in general.

Closes #11138

leafty/oh-my-zsh,sankara/oh-my-zsh,jhannah/oh-my-zsh,lesterchan/oh-my-zsh,ShadowStar/oh-my-zsh,CromFr/crom-custom-oh-my-zsh,jhannah/oh-my-zsh,nunogt/oh-my-zsh,lesterchan/oh-my-zsh,upperbounds/oh-my-zsh,erwinvaneijk/oh-my-zsh,matthewberryman/oh-my-zsh,cristianstu/oh-my-zsh,aaronbieber/oh-my-zsh,aaronbieber/oh-my-zsh,codingjester/oh-my-zsh,jevinskie/oh-my-zsh,nunogt/oh-my-zsh,tonydeng/oh-my-zsh,codingjester/oh-my-zsh,RavuAlHemio/oh-my-zsh,ohlol/oh-my-zsh,cristianstu/oh-my-zsh,upperbounds/oh-my-zsh,sankara/oh-my-zsh,matthewberryman/oh-my-zsh,jevinskie/oh-my-zsh,leafty/oh-my-zsh,eiginn/oh-my-zsh,tonydeng/oh-my-zsh,erwinvaneijk/oh-my-zsh,eiginn/oh-my-zsh,ohlol/oh-my-zsh,RavuAlHemio/oh-my-zsh,ShadowStar/oh-my-zsh,CromFr/crom-custom-oh-my-zsh