blame: Automatically increase commit abbreviation length

Ensure commit object names are unique by extending the default
abbreviation as long as necessary. This allows `jgit blame` to
more closely match the formatted output of `git blame` on large
histories like Gerrit Code Review's ReceiveCommits.java file.

Change-Id: I5f7c4855769ee9dcba973389df9e109005dcdb5b

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Blame.java

@@ -143,6 +143,7 @@ class Blame extends TextBuiltin {
 			revision = null;
 		}
 
+		boolean autoAbbrev = abbrev == 0;
 		if (abbrev == 0)
 			abbrev = db.getConfig().getInt("core", "abbrev", 7); //$NON-NLS-1$ //$NON-NLS-2$
 		if (!showBlankBoundary)
@@ -156,6 +157,7 @@ class Blame extends TextBuiltin {
 			dateFmt = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss ZZZZ"); //$NON-NLS-1$
 
 		BlameGenerator generator = new BlameGenerator(db, file);
+		RevFlag scanned = generator.newFlag("SCANNED"); //$NON-NLS-1$
 		reader = db.newObjectReader();
 		try {
 			generator.setTextComparator(comparator);
@@ -198,9 +200,17 @@ class Blame extends TextBuiltin {
 			int pathWidth = 1;
 			int maxSourceLine = 1;
 			for (int line = begin; line < end; line++) {
+				RevCommit c = blame.getSourceCommit(line);
+				if (c != null && !c.has(scanned)) {
+					c.add(scanned);
+					if (autoAbbrev)
+						abbrev = Math.max(abbrev, uniqueAbbrevLen(c));
 					authorWidth = Math.max(authorWidth, author(line).length());
 					dateWidth = Math.max(dateWidth, date(line).length());
 					pathWidth = Math.max(pathWidth, path(line).length());
+				}
+				while (line + 1 < end && blame.getSourceCommit(line + 1) == c)
+					line++;
 				maxSourceLine = Math.max(maxSourceLine, blame.getSourceLine(line));
 			}
 
@@ -232,6 +242,10 @@ class Blame extends TextBuiltin {
 		}
 	}
 
+	private int uniqueAbbrevLen(RevCommit commit) throws IOException {
+		return reader.abbreviate(commit, abbrev).length();
+	}
+
 	private void parseLineRangeOption() {
 		String beginStr, endStr;
 		if (rangeString.startsWith("/")) { //$NON-NLS-1$

org.eclipse.jgit/src/org/eclipse/jgit/blame/BlameGenerator.java

@@ -422,6 +422,18 @@ public class BlameGenerator {
 		return this;
 	}
 
+	/**
+	 * Allocate a new RevFlag for use by the caller.
+	 *
+	 * @param name
+	 *            unique name of the flag in the blame context.
+	 * @return the newly allocated flag.
+	 * @since 3.4
+	 */
+	public RevFlag newFlag(String name) {
+		return revPool.newFlag(name);
+	}
+
 	/**
 	 * Execute the generator in a blocking fashion until all data is ready.
 	 *