respect core.hooksPath, falling back to .git/hooks

Since v2.9.0, Git knows about the config variable core.hookspath
that allows overriding the path to the directory containing the
Git hooks.

Since v2.10.0, the `--git-path` option respects that config
variable, too, so we may just as well use that command.

For Git versions older than v2.5.0 (which was the first version to
support the `--git-path` option for the `rev-parse` command), we
simply fall back to the previous code.

This fixes #1755

Initial-patch-by: Philipp Gortan <philipp@gortan.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

git-gui/git-gui.sh

@@ -682,7 +682,11 @@ proc git_write {args} {
 }
 
 proc githook_read {hook_name args} {
-	set pchook [gitdir hooks $hook_name]
+	if {[package vcompare $::_git_version 2.5.0] >= 0} {
+		set pchook [git rev-parse --git-path "hooks/$hook_name"]
+	} else {
+		set pchook [gitdir hooks $hook_name]
+	}
 	lappend args 2>@1
 
 	# On Windows [file executable] might lie so we need to ask