Add helper aliases

[salcode]

While working in #65 and the complexity there, it occurred that it may be helpful to have a number of small commands, which we can then use when building bigger commands.

e.g.

• get last branch name
• get tracking remote origin based on local branch name
• get tracking remote branch based on local branch name
• check if branch has tracking remote

[salcode]

The following all seem like they could be helpful. We're currently prefixing these since the thought is these wouldn't generally be used directly (and if we want to use them directly, we may want to create a shorter alias to them).

FeCurrentBranchName                   = rev-parse --abbrev-ref HEAD
FePreviousBranchName                  = rev-parse --symbolic-full-name --abbrev-ref @{-1}

# These each take one parameter (`$1`), which is the local branch name we want to use.
FeTrackingRemoteByLocalBranchName     = "!f() { git "config branch.$1.remote"; }; f"
FeTrackingBranchNameByLocalBranchName = "!f() { git "config branch.$1.merge"; }; f"