t/README: Document the predefined test prerequisites

The README for the test library suggested that you grep the
test-lib.sh for test_set_prereq to see what the preset prerequisites
were.

Remove that bit, and write a section explaining all the preset
prerequisites. Most of the text was lifted from from Junio C Hamano
and Johannes Sixt, See the "Tests in Cygwin" thread in May 2009 for
the originals:

    http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118385
    http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118434

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -410,11 +410,12 @@ library for your script to use.
  - test_set_prereq SOME_PREREQ
 
    Set a test prerequisite to be used later with test_have_prereq. The
-   test-lib will set some prerequisites for you, e.g. PERL and PYTHON
-   which are derived from ./GIT-BUILD-OPTIONS (grep test_set_prereq
-   test-lib.sh for more). Others you can set yourself and use later
-   with either test_have_prereq directly, or the three argument
-   invocation of test_expect_success and test_expect_failure.
+   test-lib will set some prerequisites for you, see the
+   "Prerequisites" section below for a full list of these.
+
+   Others you can set yourself and use later with either
+   test_have_prereq directly, or the three argument invocation of
+   test_expect_success and test_expect_failure.
 
  - test_have_prereq SOME PREREQ
 
@@ -487,6 +488,40 @@ library for your script to use.
 		...
 	'
 
+Prerequisites
+-------------
+
+These are the prerequisites that the test library predefines with
+test_have_prereq.
+
+See the prereq argument to the test_* functions in the "Test harness
+library" section above and the "test_have_prereq" function for how to
+use these, and "test_set_prereq" for how to define your own.
+
+ - PERL & PYTHON
+
+   Git wasn't compiled with NO_PERL=YesPlease or
+   NO_PYTHON=YesPlease. Wrap any tests that need Perl or Python in
+   these.
+
+ - POSIXPERM
+
+   The filesystem supports POSIX style permission bits.
+
+ - BSLASHPSPEC
+
+   Backslashes in pathspec are not directory separators. This is not
+   set on Windows. See 6fd1106a for details.
+
+ - EXECKEEPSPID
+
+   The process retains the same pid across exec(2). See fb9a2bea for
+   details.
+
+ - SYMLINKS
+
+   The filesystem we're on supports symbolic links. E.g. a FAT
+   filesystem doesn't support these. See 704a3143 for details.
 
 Tips for Writing Tests
 ----------------------