zsh completion: remove computed default values from help texts
The default encoding and encoding mode are computed at runtime and can
vary from system to system. The two remaining default values in the
completion help texts (number of directories to strip on import and
default bundle name) are fixed in the source code.
merge: remove last traces of fastforward merging
bbc97d419b16 introduced fast-forward branch merging.
66ed92ed115a partly removed it again.
The fastforward variable would always be false if branchmerge was false, so the
fastforward variable makes no difference and we can remove it completely.