git-gui: Unify wording to say "to stage" instead of "to add"

Also, the warning message when clicking "Reset" is adapted to
the wording "Reset" rather than a confusion "Cancel commit?".

Signed-off-by: Christian Stimming <stimming@tuhh.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/lib/commit.tcl b/lib/commit.tcl
index 1f5c2c3..f857a2f 100644
--- a/lib/commit.tcl
+++ b/lib/commit.tcl
@@ -153,7 +153,7 @@
 		U? {
 			error_popup "Unmerged files cannot be committed.
 
-File [short_path $path] has merge conflicts.  You must resolve them and add the file before committing.
+File [short_path $path] has merge conflicts.  You must resolve them and stage the file before committing.
 "
 			unlock_index
 			return
@@ -169,7 +169,7 @@
 	if {!$files_ready && ![string match *merge $curType]} {
 		info_popup {No changes to commit.
 
-You must add at least 1 file before you can commit.
+You must stage at least 1 file before you can commit.
 }
 		unlock_index
 		return