[REPL] fix computation of startpos for path completions (#52009)

Fixes #51985

Ensure that the REPL completions escape and unescape text correctly,
using the correct functions, and accounting for exactly what the user
has currently typed.

The old broken method is left around for Pkg, since it has an
over-reliance on it returning incorrect answers. Once Pkg is fixed, we
can delete that code.

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

base/shell.jl

@@ -18,12 +18,11 @@ end
 
 function shell_parse(str::AbstractString, interpolate::Bool=true;
                      special::AbstractString="", filename="none")
-    s = SubString(str, firstindex(str))
+    last_arg = firstindex(str) # N.B.: This is used by REPLCompletions
+    s = SubString(str, last_arg)
     s = rstrip_shell(lstrip(s))
 
-    # N.B.: This is used by REPLCompletions
-    last_parse = 0:-1
-    isempty(s) && return interpolate ? (Expr(:tuple,:()),last_parse) : ([],last_parse)
+    isempty(s) && return interpolate ? (Expr(:tuple,:()), last_arg) : ([], last_arg)
 
     in_single_quotes = false
     in_double_quotes = false
@@ -32,6 +31,7 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
     arg = []
     i = firstindex(s)
     st = Iterators.Stateful(pairs(s))
+    update_last_arg = false # true after spaces or interpolate
 
     function push_nonempty!(list, x)
         if !isa(x,AbstractString) || !isempty(x)
@@ -54,6 +54,7 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
     for (j, c) in st
         j, c = j::Int, c::C
         if !in_single_quotes && !in_double_quotes && isspace(c)
+            update_last_arg = true
             i = consume_upto!(arg, s, i, j)
             append_2to1!(args, arg)
             while !isempty(st)
@@ -77,12 +78,17 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
                 # use parseatom instead of parse to respect filename (#28188)
                 ex, j = Meta.parseatom(s, stpos, filename=filename)
             end
-            last_parse = (stpos:prevind(s, j)) .+ s.offset
-            push_nonempty!(arg, ex)
+            last_arg = stpos + s.offset
+            update_last_arg = true
+            push!(arg, ex)
             s = SubString(s, j)
             Iterators.reset!(st, pairs(s))
             i = firstindex(s)
         else
+            if update_last_arg
+                last_arg = i + s.offset
+                update_last_arg = false
+            end
             if !in_double_quotes && c == '\''
                 in_single_quotes = !in_single_quotes
                 i = consume_upto!(arg, s, i, j)
@@ -124,14 +130,14 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
     push_nonempty!(arg, s[i:end])
     append_2to1!(args, arg)
 
-    interpolate || return args, last_parse
+    interpolate || return args, last_arg
 
     # construct an expression
     ex = Expr(:tuple)
     for arg in args
         push!(ex.args, Expr(:tuple, arg...))
     end
-    return ex, last_parse
+    return ex, last_arg
 end
 
 function shell_split(s::AbstractString)
@@ -216,7 +222,7 @@ function print_shell_escaped_posixly(io::IO, args::AbstractString...)
         function isword(c::AbstractChar)
             if '0' <= c <= '9' || 'a' <= c <= 'z' || 'A' <= c <= 'Z'
                 # word characters
-            elseif c == '_' || c == '/' || c == '+' || c == '-'
+            elseif c == '_' || c == '/' || c == '+' || c == '-' || c == '.'
                 # other common characters
             elseif c == '\''
                 have_single = true