Fix context.(Global)IsSet to respect environment variables

This appeared to be the least messy approach to hack in support for
IsSet also checking environment variables to see if a particular
cli.Flag was set without making backwards incompatible changes to the
interface.

I intend to fix this more properly in v2, probably by adding another
method to the cli.Flag interface to push the responsibility down as it
occurred to me that it was really the `Flag`s themselves that offer
support for configuration via the environment as opposed to the
`context` or other supporting structures. This opens the door for the
anything implementing the `Flag` interface to have additional sources of
input while still supporting `context.IsSet`.

context_test.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"flag"
+	"os"
 	"testing"
 	"time"
 )
@@ -180,6 +181,33 @@ func TestContext_IsSet(t *testing.T) {
 	expect(t, c.IsSet("myflagGlobal"), false)
 }
 
+// XXX Corresponds to hack in context.IsSet for flags with EnvVar field
+// Should be moved to `flag_test` in v2
+func TestContext_IsSet_fromEnv(t *testing.T) {
+	var timeoutIsSet, tIsSet, noEnvVarIsSet, nIsSet bool
+
+	os.Clearenv()
+	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
+	a := App{
+		Flags: []Flag{
+			Float64Flag{Name: "timeout, t", EnvVar: "APP_TIMEOUT_SECONDS"},
+			Float64Flag{Name: "no-env-var, n"},
+		},
+		Action: func(ctx *Context) error {
+			timeoutIsSet = ctx.IsSet("timeout")
+			tIsSet = ctx.IsSet("t")
+			noEnvVarIsSet = ctx.IsSet("no-env-var")
+			nIsSet = ctx.IsSet("n")
+			return nil
+		},
+	}
+	a.Run([]string{"run"})
+	expect(t, timeoutIsSet, true)
+	expect(t, tIsSet, true)
+	expect(t, noEnvVarIsSet, false)
+	expect(t, nIsSet, false)
+}
+
 func TestContext_GlobalIsSet(t *testing.T) {
 	set := flag.NewFlagSet("test", 0)
 	set.Bool("myflag", false, "doc")
@@ -199,6 +227,38 @@ func TestContext_GlobalIsSet(t *testing.T) {
 	expect(t, c.GlobalIsSet("bogusGlobal"), false)
 }
 
+// XXX Corresponds to hack in context.IsSet for flags with EnvVar field
+// Should be moved to `flag_test` in v2
+func TestContext_GlobalIsSet_fromEnv(t *testing.T) {
+	var timeoutIsSet, tIsSet, noEnvVarIsSet, nIsSet bool
+
+	os.Clearenv()
+	os.Setenv("APP_TIMEOUT_SECONDS", "15.5")
+	a := App{
+		Flags: []Flag{
+			Float64Flag{Name: "timeout, t", EnvVar: "APP_TIMEOUT_SECONDS"},
+			Float64Flag{Name: "no-env-var, n"},
+		},
+		Commands: []Command{
+			{
+				Name: "hello",
+				Action: func(ctx *Context) error {
+					timeoutIsSet = ctx.GlobalIsSet("timeout")
+					tIsSet = ctx.GlobalIsSet("t")
+					noEnvVarIsSet = ctx.GlobalIsSet("no-env-var")
+					nIsSet = ctx.GlobalIsSet("n")
+					return nil
+				},
+			},
+		},
+	}
+	a.Run([]string{"run", "hello"})
+	expect(t, timeoutIsSet, true)
+	expect(t, tIsSet, true)
+	expect(t, noEnvVarIsSet, false)
+	expect(t, nIsSet, false)
+}
+
 func TestContext_NumFlags(t *testing.T) {
 	set := flag.NewFlagSet("test", 0)
 	set.Bool("myflag", false, "doc")