Git.pm: Handle failed commands' output

Currently if an external command returns error exit code, a generic exception
is thrown and there is no chance for the caller to retrieve the command's
output.

This patch introduces a Git::Error::Command exception class which is thrown
in this case and contains both the error code and the captured command output.
You can use the new git_cmd_try statement to fatally catch the exception
while producing a user-friendly message.

It also adds command_close_pipe() for easier checking of exit status of
a command we have just a pipe handle of. It has partial forward dependency
on the next patch, but basically only in the area of documentation.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-fmt-merge-msg.perl

@@ -7,6 +7,7 @@
 
 use strict;
 use Git;
+use Error qw(:try);
 
 my $repo = Git->repository();
 
@@ -31,7 +32,17 @@ sub andjoin {
 }
 
 sub repoconfig {
-	my ($val) = $repo->command_oneline('repo-config', '--get', 'merge.summary');
+	my $val;
+	try {
+		$val = $repo->command_oneline('repo-config', '--get', 'merge.summary');
+	} catch Git::Error::Command with {
+		my ($E) = shift;
+		if ($E->value() == 1) {
+			return undef;
+		} else {
+			throw $E;
+		}
+	};
 	return $val;
 }