git-gui: Add option for changing the width of the commit message text box

The width of the commit message text area is currently hard-coded
to 75 characters. This value might be not optimal for some projects.
For instance users who would like to generate GNU-style ChangeLog
file from git commit message might prefer commit messages of width
no longer than 70 characters.

This patch adds a global and per repository option "Commit Message
Text Width", which could be used to change the width of the commit
message text area.

Signed-off-by: Adam Piątyszek <ediap@users.sourceforge.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/lib/option.tcl b/lib/option.tcl
index ea80df0..9270512 100644
--- a/lib/option.tcl
+++ b/lib/option.tcl
@@ -124,6 +124,7 @@
 		{b gui.pruneduringfetch {mc "Prune Tracking Branches During Fetch"}}
 		{b gui.matchtrackingbranch {mc "Match Tracking Branches"}}
 		{i-0..99 gui.diffcontext {mc "Number of Diff Context Lines"}}
+		{i-0..99 gui.commitmsgwidth {mc "Commit Message Text Width"}}
 		{t gui.newbranchtemplate {mc "New Branch Name Template"}}
 		} {
 		set type [lindex $option 0]