Implement line-history search (git log -L)

This is a rewrite of much of Bo's work, mainly in an effort to split
it into smaller, easier to understand routines.

The algorithm is built around the struct range_set, which encodes a
series of line ranges as intervals [a,b).  This is used in two
contexts:

* A set of lines we are tracking (which will change as we dig through
  history).
* To encode diffs, as pairs of ranges.

The main routine is range_set_map_across_diff().  It processes the
diff between a commit C and some parent P.  It determines which diff
hunks are relevant to the ranges tracked in C, and computes the new
ranges for P.

The algorithm is then simply to process history in topological order
from newest to oldest, computing ranges and (partial) diffs.  At
branch points, we need to merge the ranges we are watching.  We will
find that many commits do not affect the chosen ranges, and mark them
TREESAME (in addition to those already filtered by pathspec limiting).
Another pass of history simplification then gets rid of such commits.

This is wired as an extra filtering pass in the log machinery.  This
currently only reduces code duplication, but should allow for other
simplifications and options to be used.

Finally, we hook a diff printer into the output chain.  Ideally we
would wire directly into the diff logic, to optionally use features
like word diff.  However, that will require some major reworking of
the diff chain, so we completely replace the output with our own diff
for now.

As this was a GSoC project, and has quite some history by now, many
people have helped.  In no particular order, thanks go to

  Jakub Narebski <jnareb@gmail.com>
  Jens Lehmann <Jens.Lehmann@web.de>
  Jonathan Nieder <jrnieder@gmail.com>
  Junio C Hamano <gitster@pobox.com>
  Ramsay Jones <ramsay@ramsay1.demon.co.uk>
  Will Palmer <wmpalmer@gmail.com>

Apologies to everyone I forgot.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/t/t4211/expect.vanishes-early b/t/t4211/expect.vanishes-early
new file mode 100644
index 0000000..1f7cd06
--- /dev/null
+++ b/t/t4211/expect.vanishes-early
@@ -0,0 +1,39 @@
+commit 4659538844daa2849b1a9e7d6fadb96fcd26fc83
+Author: Thomas Rast <trast@student.ethz.ch>
+Date:   Thu Feb 28 10:48:43 2013 +0100
+
+    change back to complete line
+
+diff --git a/a.c b/a.c
+--- a/a.c
++++ b/a.c
+@@ -22,1 +24,1 @@
+-}
+\ No newline at end of file
++/* incomplete lines are bad! */
+
+commit 100b61a6f2f720f812620a9d10afb3a960ccb73c
+Author: Thomas Rast <trast@student.ethz.ch>
+Date:   Thu Feb 28 10:48:10 2013 +0100
+
+    change to an incomplete line at end
+
+diff --git a/a.c b/a.c
+--- a/a.c
++++ b/a.c
+@@ -22,1 +22,1 @@
+-}
++}
+\ No newline at end of file
+
+commit de4c48ae814792c02a49c4c3c0c757ae69c55f6a
+Author: Thomas Rast <trast@student.ethz.ch>
+Date:   Thu Feb 28 10:44:48 2013 +0100
+
+    initial
+
+diff --git a/a.c b/a.c
+--- /dev/null
++++ b/a.c
+@@ -0,0 +20,1 @@
++}