environment: Document that '--manifest' can be repeated.

* tests/guix-environment.sh: Test 'guix environment' with two '-m' options.
* doc/guix.texi (Invoking guix environment): Explain that '-m' can be
passed multiple times.

doc/guix.texi

@@ -4802,7 +4802,8 @@ As an example, @var{file} might contain a definition like this
 @item --manifest=@var{file}
 @itemx -m @var{file}
 Create an environment for the packages contained in the manifest object
-returned by the Scheme code in @var{file}.
+returned by the Scheme code in @var{file}.  This option can be repeated
+several times, in which case the manifests are concatenated.
 
 This is similar to the same-named option in @command{guix package}
 (@pxref{profile-manifest, @option{--manifest}}) and uses the same

tests/guix-environment.sh

@@ -84,6 +84,29 @@ echo "(use-modules (guix profiles) (gnu packages bootstrap))
 guix environment --bootstrap --manifest=$tmpdir/manifest.scm --pure \
      -- "$SHELL" -c 'test -f "$GUIX_ENVIRONMENT/bin/guile"'
 
+# Make sure '--manifest' can be specified multiple times.
+cat > "$tmpdir/manifest2.scm" <<EOF
+(use-modules (guix) (guix profiles)
+             (guix build-system trivial)
+             (gnu packages bootstrap))
+
+(packages->manifest
+ (list (package
+         (inherit %bootstrap-guile)
+         (name "eliug")
+         (build-system trivial-build-system)
+         (arguments
+          (quasiquote
+           (#:guile ,%bootstrap-guile
+            #:builder
+            (begin
+              (mkdir %output)
+              (mkdir (string-append %output "/eliug")))))))))
+EOF
+guix environment --bootstrap -m "$tmpdir/manifest.scm" \
+     -m "$tmpdir/manifest2.scm" --pure \
+     -- "$SHELL" -c 'test -f "$GUIX_ENVIRONMENT/bin/guile" && test -d "$GUIX_ENVIRONMENT/eliug"'
+
 # Make sure '-r' works as expected.
 rm -f "$gcroot"
 expected="`guix environment --bootstrap --ad-hoc guile-bootstrap \