cloud9にての質問

cloud9にて『$ git diff』と入力したところ、なにも表示されないのが正解らしいのですが下記の表示が出てきました。どうしたらいいでしょうか？
warning: Not a git repository. Use --no-index to compare two paths outside a working tree
usage: git diff --no-index [<options>] <path> <path>

Diff output format options
-p, --patch generate patch
-s, --no-patch suppress diff output
-u generate patch
-U, --unified[=<n>] generate diffs with <n> lines context
-W, --function-context
generate diffs with <n> lines context
--raw generate the diff in raw format
--patch-with-raw synonym for '-p --raw'
--patch-with-stat synonym for '-p --stat'
--numstat machine friendly --stat
--shortstat output only the last line of --stat
-X, --dirstat[=<param1,param2>...]
output the distribution of relative amount of changes for each sub-directory
--cumulative synonym for --dirstat=cumulative
--dirstat-by-file[=<param1,param2>...]
synonym for --dirstat=files,param1,param2...
--check warn if changes introduce conflict markers or whitespace errors
--summary condensed summary such as creations, renames and mode changes
--name-only show only names of changed files
--name-status show only names and status of changed files
--stat[=<width>[,<name-width>[,<count>]]]
generate diffstat
--stat-width <width> generate diffstat with a given width
--stat-name-width <width>
generate diffstat with a given name width
--stat-graph-width <width>
generate diffstat with a given graph width
--stat-count <count> generate diffstat with limited lines
--compact-summary generate compact summary in diffstat
--binary output a binary diff that can be applied
--full-index show full pre- and post-image object names on the "index" lines
--color[=<when>] show colored diff
--ws-error-highlight <kind>
highlight whitespace errors in the 'context', 'old' or 'new' lines in the diff
-z do not munge pathnames and use NULs as output field terminators in --raw or --numstat
--abbrev[=<n>] use <n> digits to display object names
--src-prefix <prefix>
show the given source prefix instead of "a/"
--dst-prefix <prefix>
show the given destination prefix instead of "b/"
--line-prefix <prefix>
prepend an additional prefix to every line of output
--no-prefix do not show any source or destination prefix
--inter-hunk-context <n>
show context between diff hunks up to the specified number of lines
--output-indicator-new <char>
specify the character to indicate a new line instead of '+'
--output-indicator-old <char>
specify the character to indicate an old line instead of '-'
--output-indicator-context <char>
specify the character to indicate a context instead of ' '

Diff rename options
-B, --break-rewrites[=<n>[/<m>]]
break complete rewrite changes into pairs of delete and create
-M, --find-renames[=<n>]
detect renames
-D, --irreversible-delete
omit the preimage for deletes
-C, --find-copies[=<n>]
detect copies
--find-copies-harder use unmodified files as source to find copies
--no-renames disable rename detection
--rename-empty use empty blobs as rename source
--follow continue listing the history of a file beyond renames
-l <n> prevent rename/copy detection if the number of rename/copy targets exceeds given limit

Diff algorithm options
--minimal produce the smallest possible diff
-w, --ignore-all-space
ignore whitespace when comparing lines
-b, --ignore-space-change
ignore changes in amount of whitespace
--ignore-space-at-eol
ignore changes in whitespace at EOL
--ignore-cr-at-eol ignore carrier-return at the end of line
--ignore-blank-lines ignore changes whose lines are all blank
-I, --ignore-matching-lines <regex>
ignore changes whose all lines match <regex>
--indent-heuristic heuristic to shift diff hunk boundaries for easy reading
--patience generate diff using the "patience diff" algorithm
--histogram generate diff using the "histogram diff" algorithm
--diff-algorithm <algorithm>
choose a diff algorithm
--anchored <text> generate diff using the "anchored diff" algorithm
--word-diff[=<mode>] show word diff, using <mode> to delimit changed words
--word-diff-regex <regex>
use <regex> to decide what a word is
--color-words[=<regex>]
equivalent to --word-diff=color --word-diff-regex=<regex>
--color-moved[=<mode>]
moved lines of code are colored differently
--color-moved-ws <mode>
how white spaces are ignored in --color-moved

Other diff options
--relative[=<prefix>]
when run from subdir, exclude changes outside and show relative paths
-a, --text treat all files as text
-R swap two inputs, reverse the diff
--exit-code exit with 1 if there were differences, 0 otherwise
--quiet disable all output of the program
--ext-diff allow an external diff helper to be executed
--textconv run external text conversion filters when comparing binary files
--ignore-submodules[=<when>]
ignore changes to submodules in the diff generation
--submodule[=<format>]
specify how differences in submodules are shown
--ita-invisible-in-index
hide 'git add -N' entries from the index
--ita-visible-in-index
treat 'git add -N' entries as real in the index
-S <string> look for differences that change the number of occurrences of the specified string
-G <regex> look for differences that change the number of occurrences of the specified regex
--pickaxe-all show all changes in the changeset with -S or -G
--pickaxe-regex treat <string> in -S as extended POSIX regular expression
-O <file> control the order in which files appear in the output
--rotate-to <path> show the change in the specified path first
--skip-to <path> skip the output to the specified path
--find-object <object-id>
look for differences that change the number of occurrences of the specified object
--diff-filter [(A|C|D|M|R|T|U|X|B)...[*]]
select files by diff type
--output <file> output to a specific file