git-gui (Windows): use git-bash.exe if it is available

Git for Windows 2.x ships with an executable that starts the Git Bash
with all the environment variables and what not properly set up. It is
also adjusted according to the Terminal emulator option chosen when
installing Git for Windows (while `bash.exe --login -i` would always
launch with Windows' default console).

So let's use that executable (usually C:\Program Files\Git\git-bash.exe)
instead of `bash.exe --login -i` if its presence was detected.

This fixes git-for-windows/git#490

Signed-off-by: Thomas Kläger <thomas.klaeger@10a.ch>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

git-gui.sh

@@ -2736,10 +2736,18 @@ 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 &]
+	} else {
+		set cmdLine [list "Git Bash" bash --login -l &]
+	}
 	.mbar.repository add command \
 		-label [mc "Git Bash"] \
-		-command {eval exec [auto_execok start] \
-					  [list "Git Bash" bash --login -l &]}
+		-command {eval exec [auto_execok start] $cmdLine}
 }
 
 if {[is_Windows] || ![is_bare]} {