[PATCH 03/20] blame doc: explicitly note how --abbrev=40 gives 39 chars

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

In a later change I'm adding stress testing of the commit abbreviation
as it relates to git-blame and others, and initially thought that the
inability to extract full SHA-1s from the non-"--porcelain" output was
a bug.

In hindsight I could have read the existing paragraph more carefully,
but let's make this clearer by explicitly stating this limitation of
--abbrev as it relates to git-blame, it is not shared by any other
command that supports core.abbrev or --abbrev.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-blame.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/git-blame.txt b/Documentation/git-blame.txt
index 16323eb80e..7b562494ac 100644
--- a/Documentation/git-blame.txt
+++ b/Documentation/git-blame.txt
@@ -88,6 +88,11 @@ include::blame-options.txt[]
 	Instead of using the default 7+1 hexadecimal digits as the
 	abbreviated object name, use <n>+1 digits. Note that 1 column
 	is used for a caret to mark the boundary commit.
++
+Because of this UI design, the only way to get the full SHA-1 of the
+boundary commit is to use the `--porcelain` format. With `--abbrev=40`
+only 39 characters of the boundary SHA-1 will be emitted, since one
+will be used for the caret to mark the boundary.
 
 
 THE PORCELAIN FORMAT
-- 
2.17.0.290.gded63e768a