git-gui: cleanup git-bash menu item

git-gui on Git for Windows creates a menu item to start a git-bash
session for the current repository. This menu-item works as desired when
git-gui is installed in the Git for Windows (g4w) distribution, but
not when run from a different location such as normally done in
development. The reason is that git-bash's location is known to be
'/git-bash' in the Unix pathname space known to MSYS, but this is not
known in the Windows pathname space. Instead, git-gui derives a pathname
for git-bash assuming it is at a known relative location.

If git-gui is run from a different directory than assumed in g4w, the
relative location changes, and git-gui resorts to running a generic bash
login session in a Windows console.

But, the MSYS system underlying Git for Windows includes the 'cygpath'
utility to convert between Unix and Windows pathnames. Let's use this so
git-bash's Windows pathname is determined directly from /git-bash.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Author: mark987

git-gui.sh

@@ -2759,17 +2759,16 @@ if {![is_bare]} {
 
 if {[is_Windows]} {
 	# Use /git-bash.exe if available
-	set normalized [file normalize $::argv0]
-	regsub "/mingw../libexec/git-core/git-gui$" \
-		$normalized "/git-bash.exe" cmdLine
-	if {$cmdLine != $normalized && [file exists $cmdLine]} {
-		set cmdLine [list "Git Bash" $cmdLine &]
+	set _git_bash [exec cygpath -m /git-bash.exe]
+	if {[file executable $_git_bash]} {
+		set _bash_cmdline [list "Git Bash" $_git_bash &]
 	} else {
-		set cmdLine [list "Git Bash" bash --login -l &]
+		set _bash_cmdline [list "Git Bash" bash --login -l &]
 	}
 	.mbar.repository add command \
 		-label [mc "Git Bash"] \
-		-command {eval exec [list [_which cmd] /c start] $cmdLine}
+		-command {eval exec [list [_which cmd] /c start] $_bash_cmdline}
+	unset _git_bash
 }
 
 if {[is_Windows] || ![is_bare]} {