[PATCH] p7519: allow running without watchman prereq

p7519 measures the performance of the fsmonitor code. To do this, it uses the installed copy of Watchman. If Watchman isn't installed, a noop integration script is installed in its place.

When in the latter mode, it is expected that the script should not write a "last update token": in fact, it doesn't write anything at all since the script is blank.

Commit 33226af42b (t/perf/fsmonitor: improve error message if typoing hook name, 2020-10-26) made sure that running 'git update-index --fsmonitor' did not write anything to stderr, but this is not the case when using the empty Watchman script, since Git will complain that:

    $ which watchman
    watchman not found
    $ cat .git/hooks/fsmonitor-empty
    $ git -c core.fsmonitor=.git/hooks/fsmonitor-empty update-index --fsmonitor
    warning: Empty last update token.

Prior to 33226af42b, the output wasn't checked at all, which allowed this noop mode to work. But, 33226af42b breaks p7519 when running it without a 'watchman(1)' on your system.

Handle this by only checking that the stderr is empty only when running with a real watchman executable. Otherwise, assert that the error message is the expected one when running in the noop mode.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
Noticed while running the perf tests on v2.30.0 on a system that happens
to not have Watchman installed.

I could do without the 'else' block here, but it seems straightforward enough. Likewise, the "ensure no silent error" comment could certainly be removed (since 'test_must_be_empty error' makes what is happening quite clear), but the fix pursued here is minimal.

 t/perf/p7519-fsmonitor.sh | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)