t/perf: fix test_export() failure with BSD `sed`

t/perf: fix test_export() failure with BSD `sed`

test_perf() runs each test in its own subshell which makes it difficult
to persist variables between tests. test_export() addresses this
shortcoming by grabbing the values of specified variables after a test
runs but before the subshell exits, and writes those values to a file
which is loaded into the environment of subsequent tests.

To grab the values to be persisted, test_export() pipes the output of
the shell's builtin `set` command through `sed` which plucks them out
using a regular expression along the lines of `s/^(var1|var2)/.../p`.
Unfortunately, though, this use of alternation is not portable. For
instance, BSD-lineage `sed` (including macOS `sed`) does not support it
in the default "basic regular expression" mode (BRE). It may be possible
to enable "extended regular expression" mode (ERE) in some cases with
`sed -E`, however, `-E` is neither portable nor part of POSIX.

Fortunately, alternation is unnecessary in this case and can easily be
avoided, so replace it with a series of simple expressions such as
`s/^var1/.../p;s/^var2/.../p`.

While at it, tighten the expressions so they match the variable names
exactly rather than matching prefixes (i.e. use `s/^var1=/.../p`).

If the requirements of test_export() become more complex in the future,
then an alternative would be to replace `sed` with `perl` which supports
alternation on all platforms, however, the simple elimination of
alternation via multiple `sed` expressions suffices for the present.

Reported-by: Sangeeta <sangunb09@gmail.com>
Diagnosed-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>