Support for command wrapping ("aliases")

Add the --wraps option to 'complete' and 'function'. This allows a
command to (recursively) inherit the completions of a wrapped command.
Fixes #393.

When evaluating a completion, we inspect the entire "wrap chain" for a
command, i.e. we follow the sequence of wrapping until we either hit a
loop (which we silently ignore) or the end of the chain. We then
evaluate completions as if the wrapping command were substituted with
the wrapped command. Currently this only works for commands, i.e.
'complete --command gco --wraps git\ checkout' won't work (that would
seem to encroaching on abbreviations anyways). It might be useful to
show an error message for that case.

The commandline builtin reflects the commandline with the wrapped
command substituted in, so e.g. git completions (which inspect the
command line) will just work. This sort of command line munging is
also performed by 'complete -C' so it's not totally without precedent.

'alias will also now mark its generated function as wrapping the
'target.

doc_src/complete.txt

@@ -1,7 +1,7 @@
 \section complete complete - edit command specific tab-completions
 
 \subsection complete-synopsis Synopsis
-<tt>complete (-c|--command|-p|--path) COMMAND [(-s|--short-option) SHORT_OPTION] [(-l|--long-option|-o|--old-option) LONG_OPTION [(-a||--arguments) OPTION_ARGUMENTS] [(-d|--description) DESCRIPTION] </tt>
+<tt>complete (-c|--command|-p|--path) COMMAND [(-s|--short-option) SHORT_OPTION] [(-l|--long-option|-o|--old-option) LONG_OPTION [(-a||--arguments) OPTION_ARGUMENTS] [(-w|--wraps) WRAPPED_COMMAND] [(-d|--description) DESCRIPTION] </tt>
 
 \subsection complete-description Description
 
@@ -24,6 +24,7 @@ the fish manual.
 - <tt>-u</tt> or <tt>--unauthoritative</tt> implies that there may be more options than the ones specified, and that fish should not assume that options not listed are spelling errors
 - <tt>-A</tt> or <tt>--authoritative</tt> implies that there may be no more options than the ones specified, and that fish should assume that options not listed are spelling errors
 - <tt>-x</tt> or <tt>--exclusive</tt> implies both <tt>-r</tt> and <tt>-f</tt>
+- <tt>-w WRAPPED_COMMAND</tt> or <tt>--wraps=WRAPPED_COMMAND</tt> causes the specified command to inherit completions from the wrapped comamnd.
 
 Command specific tab-completions in \c fish are based on the notion
 of options and arguments. An option is a parameter which begins with a
@@ -41,6 +42,14 @@ switches may all be used multiple times to specify multiple commands
 which have the same completion or multiple switches accepted by a
 command.
 
+The \c -w or \c --wraps options causes the specified command to inherit
+completions from another command. The inheriting command is said to
+"wrap" the inherited command. The wrapping command may have its own
+completions in addition to inherited ones. A command may wrap multiple
+commands, and wrapping is transitive: if A wraps B, and B wraps C,
+then A automatically inherits all of C's completions. Wrapping can
+be removed using the \c -e or \c --erase options.
+
 When erasing completions, it is possible to either erase all
 completions for a specific command by specifying <tt>complete -e -c
 COMMAND</tt>, or by specifying a specific completion option to delete
@@ -75,3 +84,10 @@ This can be written as:
 where \c __fish_contains_opt is a function that checks the commandline
 buffer for the presence of a specified set of options.
 
+To implement an alias, use the \c -w or \c --wraps option:
+
+<tt>complete -c hub -w git</tt>
+
+Now hub inherits all of the completions from git. Note this can
+also be specified in a function declaration.
+

doc_src/function.txt

@@ -14,6 +14,7 @@ The following options are available:
 
 - <code>-a NAMES</code> or <code>--argument-names NAMES</code> assigns the value of successive command-line arguments to the names given in NAMES.
 - <code>-d DESCRIPTION</code> or \c --description=DESCRIPTION is a description of what the function does, suitable as a completion description.
+- <code>-w WRAPPED_COMMAND</code> or \c --wraps=WRAPPED_COMMAND causes the function to inherit completions from the given wrapped command. See the documentation for \c complete for more information.
 - <code>-e</code> or <code>--on-event EVENT_NAME</code> tells fish to run this function when the specified named event is emitted. Fish internally generates named events e.g. when showing the prompt.
 - <code>-j PID</code> or <code> --on-job-exit PID</code> tells fish to run this function when the job with group ID PID exits. Instead of PID, the string 'caller' can be specified. This is only legal when in a command substitution, and will result in the handler being triggered by the exit of the job which created this command substitution.
 - <code>-p PID</code> or <code> --on-process-exit PID</code> tells fish to run this function when the fish child process with process ID PID exits.