Add very simple revision comparison for git.

At this moment we do not allow comparison between arbitrary hashes,
but except GUI the code is ready.
Thanks to the powerful way of git addressing we could even ask
for comparisons like '-2 weeks back' if someone wants to play
with GuiCompareHistory.

src/VCBackend.cpp

@@ -2213,15 +2213,54 @@ void GIT::getLog(FileName const & tmpf)
 }
 
 
-bool GIT::prepareFileRevision(string const & /*revis*/, string & /*f*/)
+//at this moment we don't accept revision SHA, but just number of revision steps back
+//GUI and infrastucture needs to be changed first
+bool GIT::prepareFileRevision(string const & revis, string & f)
 {
+	// anything positive means we got hash, not "0" or minus revision
+	int rev = 1;
+
+	// hash is rarely number and should be long
+	if (isStrInt(revis) && revis.length()<20)
+		rev = convert<int>(revis);
+
+	// revision and filename
+	string pointer;
+
+	// go back for "minus" revisions
+	if (rev <= 0)
+		pointer = "HEAD~" + convert<string>(-rev);
+	// normal hash
+	else
+		pointer = revis;
+
+	pointer += ":";
+
+	if (rev <= 0)
+		if (!getFileRevisionInfo())
 			return false;
+
+	FileName tmpf = FileName::tempName("lyxvcrev_" + revis + "_");
+	if (tmpf.empty()) {
+		LYXERR(Debug::LYXVC, "Could not generate logfile " << tmpf);
+		return false;
+	}
+
+	doVCCommand("git show " + pointer + "./"
+	              + quoteName(onlyFileName(owner_->absFileName()))
+		      + " > " + quoteName(tmpf.toFilesystemEncoding()),
+		FileName(owner_->filePath()));
+	if (tmpf.isFileEmpty())
+		return false;
+
+	f = tmpf.absFileName();
+	return true;
 }
 
 
 bool GIT::prepareFileRevisionEnabled()
 {
-	return false;
+	return true;
 }
 
 

src/frontends/qt4/GuiCompareHistory.cpp

@@ -10,6 +10,7 @@
 
 #include <config.h>
 #include <support/debug.h>
+#include <limits>
 
 #include "GuiCompareHistory.h"
 
@@ -51,11 +52,22 @@ bool GuiCompareHistory::initialiseParams(std::string const &)
 	int rev=0;
 
 	string tmp;
+	bool enableBetween = true;
+	// GIT case, hash is long
+	if (revstring.length() > 20) {
+		enableBetween = false;
+		rev = numeric_limits<int>::max();
+	} else {
 		// RCS case
 		if (!isStrInt(revstring))
 			revstring = rsplit(revstring, tmp , '.' );
+		// both SVN & RCS cases
 		if (isStrInt(revstring))
 			rev = convert<int>(revstring);
+	}
+
+	// later we can provide comparison between two hashes
+	betweenrevRB->setEnabled(enableBetween);
 
 	okPB->setEnabled(rev);
 	rev1SB->setMaximum(rev);