git-gui: Fixes chooser not accepting gitfiles

Support the case where .git is a platform independent symbolic link
and not a directory. This occurs when --separate-git-dir is used when
creating the local repository to store the .git directory elsewhere.

git-gui does not support such repositories when using the repository
chooser as the test to determine that the chosen directory is a git
repository fails for such repositories.

This commit enables _is_git to read the real location from the
symbolic link file.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Remi Rampin <remirampin@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/choose_repository.tcl

@@ -339,6 +339,16 @@ method _git_init {} {
 }
 
 proc _is_git {path} {
+	if {[file isfile $path]} {
+		set fp [open $path r]
+		gets $fp line
+		close $fp
+		if {[regexp "^gitdir: (.+)$" $line line link_target]} {
+			set path [file join [file dirname $path] $link_target]
+			set path [file normalize $path]
+		}
+	}
+
 	if {[file exists [file join $path HEAD]]
 	 && [file exists [file join $path objects]]
 	 && [file exists [file join $path config]]} {