git-gui - re-enable use of hook scripts

git-gui - re-enable use of hook scripts

Earlier, commit aae9560a introduced search in $PATH to find executables
before running them, avoiding an issue where on Windows a same named
file in the current directory can be executed in preference to anything
in a directory in $PATH. This search is intended to find an absolute
path for a bare executable ( e.g, a function "foo") by finding the first
instance of "foo" in a directory given in $PATH, and this search works
correctly.  The search is explicitly avoided for an executable named
with an absolute path (e.g., /bin/sh), and that works as well.

Unfortunately, the search is also applied to commands named with a
relative path. A hook script (or executable) $HOOK is usually located
relative to the project directory as .git/hooks/$HOOK. The search for
this will generally fail as that relative path will (probably) not exist
on any directory in $PATH. This means that git hooks in general now fail
to run. Considerable mayhem could occur should a directory on $PATH be
git controlled. If such a directory includes .git/hooks/$HOOK, that
repository's $HOOK will be substituted for the one in the current
project, with unknown consequences.

This lookup failure also occurs in worktrees linked to a remote .git
directory using git-new-workdir. However, a worktree using a .git file
pointing to a separate git directory apparently avoids this: in that
case the hook command is resolved to an absolute path before being
passed down to the code introduced in aae9560a.

Fix this by replacing the test for an "absolute" pathname to a check for
a command name having more than one pathname component. This limits the
search and absolute pathname resolution to bare commands. The new test
uses tcl's "file split" command. Experiments on Linux and Windows, using
tclsh, show that command names with relative and absolute paths always
give at least two components, while a bare command gives only one.

	  Linux:   puts [file split {foo}]       ==>  foo
	  Linux:   puts [file split {/foo}]      ==>  / foo
	  Linux:   puts [file split {.git/foo}]  ==> .git foo
	  Windows: puts [file split {foo}]       ==>  foo
	  Windows: puts [file split {c:\foo}]    ==>  c:/ foo
	  Windows: puts [file split {.git\foo}]  ==> .git foo

The above results show the new test limits search and replacement
to bare commands on both Linux and Windows.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>