[PATCH] mergetools/meld: Use --help output to detect --output support

[Jonathan Nieder <jrnieder@gmail.com>]

In v1.7.7-rc0~3^2 (2011-08-19), git mergetool's "meld" support learned
to use the --output option when calling versions of meld that are
detected to support it (1.5.0 and newer, hopefully).

Alas, it misdetects old versions (before 1.1.5, 2006-06-11) of meld as
supporting the option, so on systems with such meld, instead of
getting a nice merge helper, the operator gets a dialog box with the
text "Wrong number of arguments (Got 5)".  (Version 1.1.5 is when meld
switched to using optparse.  One consequence of that change was that
errors in usage are detected and signalled through the exit status
even when --help was passed.)

Luckily there is a simpler check that is more reliable: the usage
string printed by "meld --help" reliably reflects whether --output is
supported in a given version.  Use it.

Reported-by: Jeff Epler <jepler@unpythonic.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
Junio C Hamano wrote:

> When an older meld fails when given --output for real (not with the dry
> run current code tries with --help), can we sanely detect that particular
> failure?

Unfortunately it just pops up a GUI with a modal dialog box like this:
	 ___________________________________
	|                                   |
	| Wrong number of arguments (Got 5) |
	|                                   |
	|                     [Quit] [OK]   |
	|___________________________________|

If I choose "Quit", the exit status is 0.

But how about this?  "meld --help | grep -e --output" seems to detect
support for the option reliably.  With 2>&1 on the upstream of the
pipe, this even seems futureproof. ;-)

 mergetools/meld |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mergetools/meld b/mergetools/meld
index eaa115cc..cb672a55 100644
--- a/mergetools/meld
+++ b/mergetools/meld
@@ -23,7 +23,7 @@ check_meld_for_output_version () {
 	meld_path="$(git config mergetool.meld.path)"
 	meld_path="${meld_path:-meld}"
 
-	if "$meld_path" --output /dev/null --help >/dev/null 2>&1
+	if "$meld_path" --help 2>&1 | grep -e --output >/dev/null
 	then
 		meld_has_output_option=true
 	else
-- 
1.7.9