[Fix] nvm_process_nvmrc: don't show the content of a symlinked .nvmrc

`.nvmrc` is found and read through symlinks,
so a repository that commits `.nvmrc` as a symlink to a path outside itself
(eg, `../../.npmrc`)
made a plain `nvm use`, `nvm install`, or `nvm which`
(or `nvm-exec`, or the README's auto-switching shell hook)
print that file:
the invalid `.nvmrc` message shows everything that was parsed,
even with `--silent`,
and a one-line file was taken as the version,
which is printed in the "Found" message and in later errors.

A symlinked `.nvmrc` is still followed,
so linking to another version file
(eg, `.node-version`, or a parent directory's `.nvmrc`)
keeps working,
but its content is never shown,
and it is valid only if it names a version or an existing alias,
via a new `nvm_is_version_or_alias` helper.

test/fast/Unit tests/nvm_process_nvmrc does not show a symlinked .nvmrc's content

@@ -0,0 +1,71 @@
+#!/bin/sh
+
+die () { echo "$@" ; cleanup ; exit 1; }
+
+cleanup() {
+  rm -rf "${TEST_DIR-}"
+  nvm unalias nvmrc_symlink_alias > /dev/null 2>&1
+  unset TEST_DIR
+}
+
+: nvm.sh
+\. ../../../nvm.sh
+
+\. ../../common.sh
+
+TEST_DIR="${PWD}/nvm_process_nvmrc_symlink_tmp"
+mkdir -p "${TEST_DIR}/home" "${TEST_DIR}/project/packages/a" || die 'failed to create test dirs'
+printf '//registry.npmjs.org/:_authToken=SECRET\n' > "${TEST_DIR}/home/.npmrc"
+printf 'https://user:SECRET@github.com\n' > "${TEST_DIR}/home/.git-credentials"
+
+NVMRC="${TEST_DIR}/project/.nvmrc"
+
+for TARGET in .npmrc .git-credentials; do
+  rm -f "${NVMRC}"
+  ln -s "../home/${TARGET}" "${NVMRC}" || die 'failed to create the symlink'
+
+  try_err nvm_process_nvmrc "${NVMRC}"
+  [ "${CAPTURED_EXIT_CODE}" != 0 ] || die "expected a symlink to ${TARGET} to be invalid; got exit code ${CAPTURED_EXIT_CODE}"
+  case "${CAPTURED_STDERR}" in
+    *SECRET*) die "expected a symlink to ${TARGET} to not show its content; got \`${CAPTURED_STDERR}\`" ;;
+  esac
+  [ "${CAPTURED_STDERR}" = "invalid .nvmrc! '${NVMRC}' is a symbolic link, so it must contain a version or an alias name; its content is not shown." ] || die "expected the symlink error for ${TARGET}; got \`${CAPTURED_STDERR}\`"
+
+  try nvm_process_nvmrc "${NVMRC}"
+  [ -z "${CAPTURED_STDOUT}" ] || die "expected no version from a symlink to ${TARGET}; got \`${CAPTURED_STDOUT}\`"
+done
+
+nvm alias nvmrc_symlink_alias v0.0.1 > /dev/null 2>&1 || die 'failed to create an alias'
+
+# a deliberate symlink to another version file, in or above the project, still works
+for VERSION in v20.1.0 20 'v20.0.0-rc.1' node system 'lts/*' lts/iron lts/-1 nvmrc_symlink_alias; do
+  rm -f "${NVMRC}" "${TEST_DIR}/project/packages/a/.nvmrc"
+  printf '%s # comment\n' "${VERSION}" > "${NVMRC}"
+  ln -s ../../.nvmrc "${TEST_DIR}/project/packages/a/.nvmrc" || die 'failed to create the symlink'
+
+  try nvm_process_nvmrc "${TEST_DIR}/project/packages/a/.nvmrc"
+  [ "${CAPTURED_EXIT_CODE}" = 0 ] || die "expected a symlink to an .nvmrc with \`${VERSION}\` to be valid; got exit code ${CAPTURED_EXIT_CODE}"
+  [ "${CAPTURED_STDOUT}" = "${VERSION}" ] || die "expected \`${VERSION}\` from a symlinked .nvmrc; got \`${CAPTURED_STDOUT}\`"
+done
+
+for VERSION in not_an_alias lts/Iron 'lts/' 'lts/-x'; do
+  rm -f "${TEST_DIR}/project/.node-version"
+  printf '%s\n' "${VERSION}" > "${TEST_DIR}/project/.node-version"
+  rm -f "${NVMRC}"
+  ln -s .node-version "${NVMRC}" || die 'failed to create the symlink'
+
+  try_err nvm_process_nvmrc "${NVMRC}"
+  [ "${CAPTURED_EXIT_CODE}" != 0 ] || die "expected a symlink to a file with \`${VERSION}\` to be invalid; got exit code ${CAPTURED_EXIT_CODE}"
+  case "${CAPTURED_STDERR}" in
+    *"${VERSION}"*) die "expected a symlink to a file with \`${VERSION}\` to not show its content; got \`${CAPTURED_STDERR}\`" ;;
+  esac
+done
+
+# a regular file still shows what was parsed
+rm -f "${NVMRC}"
+printf 'foo=bar\n' > "${NVMRC}"
+try_err nvm_process_nvmrc "${NVMRC}"
+[ "${CAPTURED_EXIT_CODE}" != 0 ] || die "expected an .nvmrc without a version to be invalid; got exit code ${CAPTURED_EXIT_CODE}"
+nvm_echo "${CAPTURED_STDERR}" | strip_colors | nvm_grep -q '^foo=bar$' || die "expected a regular .nvmrc's parsed content to be shown; got \`${CAPTURED_STDERR}\`"
+
+cleanup